;;; guile-bstruct -- Binary structures for Guile
;;; Copyright © 2024 David Thompson <dthompson2@worcester.edu>
;;;
;;; Licensed under the Apache License, Version 2.0 (the "License");
;;; you may not use this file except in compliance with the License.
;;; You may obtain a copy of the License at
;;;
;;;    http://www.apache.org/licenses/LICENSE-2.0
;;;
;;; Unless required by applicable law or agreed to in writing, software
;;; distributed under the License is distributed on an "AS IS" BASIS,
;;; WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
;;; See the License for the specific language governing permissions and
;;; limitations under the License.

;;; Commentary:
;;
;; A bstruct is a data type that encapsulates a bytevector and an
;; offset which interprets that bytevector based on a given layout.
;; Bstructs are useful for manipulating C structs when using the FFI,
;; or interpreting/manipulating packed binary data such as GPU vertex
;; buffers, or for data types that benefit from unboxed math
;; optimizations such as vectors and matrices.
;;
;; Inspired by guile-opengl's define-packed-struct and based on
;; "Ftypes: Structured foreign types" by Andy Keep and R. Kent Dybvig.
;;
;;     http://scheme2011.ucombinator.org/papers/Keep2011.pdf
;;
;; Features:
;;
;; - Efficiency through macro magic.  Procedural macros generate tasty
;; code for the compiler to munch on and emit efficient bytecode.
;; Runtime checks are reduced to the bare minimum.
;;
;; - Raw bytevector access mode.  Easily read/write structured data
;; from/to raw bytevectors without going through an intermediary
;; struct.  Very useful for batch processing in situations when the
;; overhead of creating wrapper structs would hinder throughput.
;;
;; - Cache friendly.  In performance sensitive code, many bstructs can
;; be stored in contiguous memory by pre-allocating a large bytevector
;; for the underlying storage.  Individual bstruct handles simply
;; point at different offsets.
;;
;;; Code:

(define-module (bstruct)
  #:use-module (ice-9 exceptions)
  #:use-module (ice-9 match)
  #:use-module (rnrs bytevectors)
  #:use-module (srfi srfi-1)
  #:use-module (srfi srfi-9)
  #:use-module (srfi srfi-11)
  #:use-module (system base target)
  #:use-module ((system foreign) #:prefix ffi:)
  #:use-module (system syntax)
  #:export (bstruct?
            bstruct-=?
            bstruct-type
            bstruct-length
            bstruct->sexp
            bstruct->pointer
            pointer->bstruct
            bstruct-wrap
            bstruct-unwrap
            bstruct-alloc
            bstruct-sizeof
            bstruct-alignof
            bstruct-ref
            bstruct-set!
            bstruct-unpack
            bstruct-pack!
            bstruct-copy
            bstruct-copy!
            define-bstruct))

(define-record-type <scalar>
  (make-scalar size alignment native? type)
  scalar?
  (size scalar-size)
  (alignment scalar-alignment)
  (native? scalar-native?) ; native endianness?
  (type scalar-type))

(define-record-type <struct-field>
  (make-struct-field name offset type)
  struct-field?
  (name struct-field-name)
  (offset struct-field-offset)
  (type struct-field-type))

(define-record-type <struct>
  (make-struct size alignment fields)
  %struct?
  (size struct-size)
  (alignment struct-alignment)
  (fields struct-fields))

(define-record-type <union-field>
  (make-union-field name type)
  union-field?
  (name union-field-name)
  (type union-field-type))

(define-record-type <union>
  (make-union size alignment fields)
  union?
  (size union-size)
  (alignment union-alignment)
  (fields union-fields))

(define-record-type <array>
  (make-array size alignment length type)
  array?
  (size array-size)
  (alignment array-alignment)
  (length array-length)
  (type array-type))

(define-record-type <pointer>
  (make-pointer size alignment type)
  pointer?
  (size pointer-size)
  (alignment pointer-alignment)
  ;; Mutable for recursive types.
  (type pointer-type set-pointer-type!))

(define-record-type <opaque>
  (make-opaque)
  opaque?)

;; TODO: functions
;; TODO: bitfields

(define (struct-field-ref struct name)
  (and=> (find (match-lambda
                 (($ <struct-field> name*)
                  (eq? name name*)))
               (struct-fields struct))
         struct-field-type))

(define (union-field-ref struct name)
  (and=> (find (match-lambda
                 (($ <union-field> name*)
                  (eq? name name*)))
               (union-fields struct))
         union-field-type))

(define (bstruct-type? obj)
  (or (scalar? obj)
      (%struct? obj)
      (union? obj)
      (array? obj)
      (pointer? obj)
      (opaque? obj)))

(define (sizeof type)
  (match type
    ((or ($ <scalar> size)
         ($ <struct> size)
         ($ <union> size)
         ($ <array> size)
         ($ <pointer> size))
     size)
    (($ <opaque>)
     (raise-exception
      (make-exception (make-exception-with-message
                       "cannot get size of opaque type")
                      (make-exception-with-origin 'sizeof)
                      (make-exception-with-irritants (list type)))))))

(define (alignof type)
  (match type
    ((or ($ <scalar> _ alignment)
         ($ <struct> _ alignment)
         ($ <union> _ alignment)
         ($ <array> _ alignment)
         ($ <pointer> _ alignment))
     alignment)
    (($ <opaque>)
     (raise-exception
      (make-exception (make-exception-with-message
                       "cannot get alignment of opaque type")
                      (make-exception-with-origin 'sizeof)
                      (make-exception-with-irritants (list type)))))))

(define-syntax-rule (assert expr who)
  (unless expr
    (raise-exception
     (make-exception (make-assertion-failure)
                     (make-exception-with-origin who)
                     (make-exception-with-irritants '(expr))))))

(define-syntax-rule (check-size i n who)
  (assert (and (>= i 0) (< i n)) who))

(define-inlinable (u64? x)
  (and (exact-integer? x) (<= 0 x (ash 1 64))))

;; Bstructs form a shallow vtable hierarchy.
(define <bstruct-descriptor>
  (make-vtable (string-append standard-vtable-fields "pwpw")
               (lambda (desc port)
                 (format port "#<bstruct-descriptor ~a>"
                         (object-address desc)))))

(define (bstruct-descriptor-name descriptor)
  (struct-ref descriptor vtable-offset-user))

(define (bstruct-descriptor-type descriptor)
  (struct-ref descriptor (+ vtable-offset-user 1)))

(define (bstruct-descriptor-sizeof descriptor)
  (sizeof (bstruct-descriptor-type descriptor)))

(define (print-bstruct bs port)
  (format port "#<~a ~a>"
          (bstruct-descriptor-name (struct-vtable bs))
          (bstruct->sexp bs)))

(define* (make-bstruct-descriptor name type #:key (printer print-bstruct))
  (assert (bstruct-type? type) 'make-bstruct-descriptor)
  (make-struct/no-tail <bstruct-descriptor>
                       (make-struct-layout "pwpwpw")
                       printer name type))

(define (bstruct-descriptor? obj)
  (and (struct? obj) (eq? (struct-vtable obj) <bstruct-descriptor>)))

(define (%bstruct? obj)
  (and (struct? obj) (bstruct-descriptor? (struct-vtable obj))))

(define (bstruct-bytevector bs)
  (assert (%bstruct? bs) 'bstruct-bytevector)
  (struct-ref bs 0))

(define (bstruct-offset bs)
  (assert (%bstruct? bs) 'bstruct-bytevector)
  (struct-ref bs 1))

(define (bstruct-length bs)
  (assert (%bstruct? bs) 'bstruct-bytevector)
  (struct-ref bs 2))

(define (bstruct-type bs)
  (assert (%bstruct? bs) 'bstruct-bytevector)
  (bstruct-descriptor-type (struct-vtable bs)))

;; Bstructs are composed of a type descriptor, a bytevector that
;; provides storage, an offset pointing to the start of the struct
;; data within that bytevector, and the number of contiguous structs
;; within.
;;
;; TODO: We could use Guile's bytevector slices here, however they
;; were only introduced in 3.0.9 so we can't rely on them.
(define (%make-bstruct descriptor bv offset n)
  (make-struct/no-tail descriptor bv offset n))

(define (make-bstruct descriptor bv offset n)
  (assert (bstruct-descriptor? descriptor) 'make-bstruct)
  (assert (bytevector? bv) 'make-bstruct)
  (assert (exact-integer? offset) 'make-bstruct)
  (assert (>= offset 0) 'make-bstruct)
  (assert (<= (+ offset (* (bstruct-descriptor-sizeof descriptor) n))
              (bytevector-length bv))
          'make-bstruct)
  (%make-bstruct descriptor bv offset n))

;; Platform ABI details
(define (non-target-endianness)
  (if (eq? (target-endianness) (endianness little))
      (endianness big)
      (endianness little)))

(define (sizeof* type)
  (match type
    ((or 'u8 's8) 1)
    ((or 'u16 's16) 2)
    ((or 'u32 's32 'f32) 4)
    ((or 'u64 's64 'f64) 8)
    (_
     (if (string=? %host-type (target-type))
         (match type
           ('uint (ffi:sizeof ffi:unsigned-int))
           ('int (ffi:sizeof ffi:int))
           ('ulong (ffi:sizeof ffi:unsigned-long))
           ('long (ffi:sizeof ffi:long))
           ('ushort (ffi:sizeof ffi:unsigned-short))
           ('short (ffi:sizeof ffi:short))
           ('size_t (ffi:sizeof ffi:size_t))
           ('ssize_t (ffi:sizeof ffi:ssize_t))
           ('ptrdiff_t (ffi:sizeof ffi:ptrdiff_t))
           ('intptr_t (ffi:sizeof ffi:intptr_t))
           ('uintptr_t (ffi:sizeof ffi:uintptr_t)))
         ;; FIXME: Fill in with proper ABI details.  We will lazily
         ;; evaluate this work when we hit a problem in a cross build.
         (match type
           ('uint 4)
           ('int 4)
           ('ulong 8)
           ('long 8)
           ('ushort 2)
           ('short 2)
           ('size_t (target-word-size))
           ('ssize_t (target-word-size))
           ('ptrdiff_t (target-word-size))
           ('intptr_t (target-word-size))
           ('uintptr_t (target-word-size)))))))

(define (alignof* type)
  (match type
    ((or 'u8 's8) 1)
    ((or 'u16 's16) 2)
    ((or 'u32 's32 'f32) 4)
    ((or 'u64 's64 'f64) 8)
    (_
     (if (string=? %host-type (target-type))
         (match type
           ('uint (ffi:sizeof ffi:unsigned-int))
           ('int (ffi:sizeof ffi:int))
           ('ulong (ffi:sizeof ffi:unsigned-long))
           ('long (ffi:sizeof ffi:long))
           ('ushort (ffi:sizeof ffi:unsigned-short))
           ('short (ffi:sizeof ffi:short))
           ('size_t (ffi:sizeof ffi:size_t))
           ('ssize_t (ffi:sizeof ffi:ssize_t))
           ('ptrdiff_t (ffi:sizeof ffi:ptrdiff_t))
           ('intptr_t (ffi:sizeof ffi:intptr_t))
           ('uintptr_t (ffi:sizeof ffi:uintptr_t))
           ('* (ffi:sizeof '*)))
         (match type
           ('uint 4)
           ('int 4)
           ('ulong 8)
           ('long 8)
           ('ushort 2)
           ('short 2)
           ('size_t (target-word-size))
           ('ssize_t (target-word-size))
           ('ptrdiff_t (target-word-size))
           ('intptr_t (target-word-size))
           ('uintptr_t (target-word-size))
           ('* (target-word-size)))))))

;; It is useful to see bstructs in s-expression form when working
;; at the REPL.
(define (bstruct->sexp bs)
  (let ((bv (bstruct-bytevector bs)))
    (let loop ((type (bstruct-type bs)) (offset (bstruct-offset bs)))
      (match type
        ((? bstruct-descriptor? desc)
         (loop (bstruct-descriptor-type desc) offset))
        (($ <scalar> _ _ native? type)
         (let ((e (if native? (target-endianness) (non-target-endianness))))
           (match type
             ('u8 (bytevector-u8-ref bv offset))
             ('s8 (bytevector-s8-ref bv offset))
             ('u16 (bytevector-u16-ref bv offset e))
             ('s16 (bytevector-s16-ref bv offset e))
             ('u32 (bytevector-u32-ref bv offset e))
             ('s32 (bytevector-s32-ref bv offset e))
             ('u64 (bytevector-u64-ref bv offset e))
             ('s64 (bytevector-s64-ref bv offset e))
             ('f32 (bytevector-ieee-single-ref bv offset e))
             ('f64 (bytevector-ieee-double-ref bv offset e))
             ('uint (bytevector-uint-ref bv offset e (ffi:sizeof ffi:unsigned-int)))
             ('int (bytevector-sint-ref bv offset e (ffi:sizeof ffi:int)))
             ('ulong (bytevector-uint-ref bv offset e (ffi:sizeof ffi:unsigned-long)))
             ('long (bytevector-sint-ref bv offset e (ffi:sizeof ffi:long)))
             ('ushort (bytevector-uint-ref bv offset e (ffi:sizeof ffi:unsigned-short)))
             ('short (bytevector-sint-ref bv offset e (ffi:sizeof ffi:short)))
             ('size_t (bytevector-uint-ref bv offset e (ffi:sizeof ffi:size_t)))
             ('ssize_t (bytevector-sint-ref bv offset e (ffi:sizeof ffi:ssize_t)))
             ('ptrdiff_t (bytevector-sint-ref bv offset e (ffi:sizeof ffi:ptrdiff_t)))
             ('intptr_t (bytevector-sint-ref bv offset e (ffi:sizeof ffi:intptr_t)))
             ('uintptr_t (bytevector-uint-ref bv offset e (ffi:sizeof ffi:uintptr_t))))))
        (($ <struct> _ _ fields)
         `(struct ,@(map (match-lambda
                           (($ <struct-field> name offset* type)
                            (list name (loop type (+ offset offset*)))))
                         fields)))
        (($ <union> _ _ fields)
         `(union ,@(map (match-lambda
                          (($ <union-field> name type)
                           (list name (loop type offset))))
                        fields)))
        (($ <array> _ _ length type)
         (let ((size (sizeof type)))
           `(array ,@(map (lambda (i)
                            (loop type (+ offset (* i size))))
                          (iota length)))))
        (($ <pointer>)
         `(* ,(bytevector-uint-ref bv offset (native-endianness)
                                   (ffi:sizeof '*))))))))

;; Macro helpers that use metadata attached to bstruct syntax
;; transformers.
(define (bstruct-descriptor-identifier? id)
  (and (identifier? id)
       (let-values (((kind val) (syntax-local-binding id)))
         (and (eq? kind 'macro)
              (procedure-property val 'bstruct?)))))

(define (non-opaque-bstruct-descriptor-identifier? id)
  (and (identifier? id)
       (let-values (((kind val) (syntax-local-binding id)))
         (and (eq? kind 'macro)
              (procedure-property val 'bstruct?)
              (not (procedure-property val 'bstruct-opaque?))))))

(define (bstruct-descriptor-identifier-size id)
  (let-values (((_ transformer) (syntax-local-binding id)))
    (procedure-property transformer 'bstruct-size)))

(define (bstruct-descriptor-identifier-alignment id)
  (let-values (((_ transformer) (syntax-local-binding id)))
    (procedure-property transformer 'bstruct-alignment)))

;; A predicate that can answer the questions:
;; 1) Is this *any kind* of bstruct?
;; 2) Is this *a specific kind* of bstruct?
(define-syntax bstruct?
  (lambda (stx)
    (syntax-case stx ()
      (x
       (identifier? #'x)
       #'(case-lambda
           ((obj) (%bstruct? obj))
           ((<type> obj)
            (match obj
              (($ <type>) #t)
              (_ #f)))))
      ((_ obj)
       #'(%bstruct? obj))
      ((_ <type> obj)
       (bstruct-descriptor-identifier? #'<type>)
       #'(match obj
           (($ <type>) #t)
           (_ #f))))))

(define-syntax bstruct-=?
  (lambda (stx)
    (syntax-case stx ()
      ((_ <type> a b)
       (bstruct-descriptor-identifier? #'<type>)
       #'(match a
           (($ <type> bv-a offset-a)
            (match b
              (($ <type> bv-b offset-b)
               (let ((n (bstruct-sizeof <type>)))
                 (let loop ((i 0))
                   (cond
                    ((= i n) #t)
                    ((= (bytevector-u8-ref bv-a (+ offset-a i))
                        (bytevector-u8-ref bv-b (+ offset-b i)))
                     (loop (+ i 1)))
                    (else #f))))))))))))

;; Create a pointer to the bstruct or some element within the bstruct.
(define-syntax bstruct->pointer
  (lambda (stx)
    (syntax-case stx ()
      ((_ (<type> i) bs)
       (bstruct-descriptor-identifier? #'<type>)
       #'(match bs
           (($ <type> bv (? u64? offset))
            (ffi:bytevector->pointer bv offset))))
      ((_ (<type> i) bs (elem ...))
       (bstruct-descriptor-identifier? #'<type>)
       #'(match bs
           (($ <type> bv (? u64? offset) (? u64? n))
            (check-size i n 'bstruct->pointer)
            (let ((offset (+ offset (* (bstruct-sizeof <type>) i))))
              (call-with-values (lambda () ((<type> offset elem ...) bv offset))
                ffi:bytevector->pointer)))))
      ((_ (<type> i) bs elem)
       #'(bstruct->pointer (<type> i) bs (elem)))
      ((_ <type> bs)
       #'(bstruct->pointer (<type> 0) bs))
      ((_ <type> bs (elem ...))
       #'(bstruct->pointer (<type> 0) bs (elem ...)))
      ((_ <type> bs elem)
       #'(bstruct->pointer (<type> 0) bs (elem))))))

(define-syntax pointer->bstruct
  (lambda (stx)
    (syntax-case stx ()
      ((_ <type> ptr n)
       (non-opaque-bstruct-descriptor-identifier? #'<type>)
       #'(let ((size (* (bstruct-sizeof <type>) n)))
           (make-bstruct <type> (ffi:pointer->bytevector ptr size) 0 n)))
      ((_ <type> ptr)
       #'(pointer->bstruct <type> ptr 1)))))

;; Wrap a bytevector in a bstruct.
(define-syntax bstruct-wrap
  (lambda (stx)
    (syntax-case stx ()
      ((_ <type> bv offset n)
       (non-opaque-bstruct-descriptor-identifier? #'<type>)
       #'(make-bstruct <type> bv offset n))
      ((_ <type> bv offset)
       #'(bstruct-wrap <type> bv offset 1)))))

;; Unwrap a bstruct to a bytevector + offset + count.
(define-syntax bstruct-unwrap
  (lambda (stx)
    (syntax-case stx ()
      ((_ <type> bs)
       (bstruct-descriptor-identifier? #'<type>)
       #`(match bs
           (($ <type> bv offset n)
            (values bv offset n)))))))

;; Size/align queries.
(define-syntax bstruct-sizeof
  (lambda (stx)
    (syntax-case stx ()
      ((_ <type>)
       (non-opaque-bstruct-descriptor-identifier? #'<type>)
       (bstruct-descriptor-identifier-size #'<type>)))))

(define-syntax bstruct-alignof
  (lambda (stx)
    (syntax-case stx ()
      ((_ <type>)
       (non-opaque-bstruct-descriptor-identifier? #'<type>)
       (bstruct-descriptor-identifier-alignment #'<type>)))))

;; 'bstruct-pack!' and 'bstruct-unpack' allow for directly
;; interpreting bytevector contents as structured data.
(define-syntax bstruct-pack!
  (lambda (stx)
    (define (flatten-elems stx)
      (append-map (lambda (stx)
                    (syntax-case stx (->)
                      ((-> root-elem sub-elems ...)
                       (identifier? #'root-elem)
                       (map (lambda (stx)
                              (syntax-case stx ()
                                (((sub-elem ...) val)
                                 #'((root-elem sub-elem ...) val))))
                            (flatten-elems #'(sub-elems ...))))
                      (((elem ...) val)
                       #'(((elem ...) val)))
                      ((elem val)
                       #'(((elem) val)))))
                  stx))
    (syntax-case stx ()
      ((_ <type> bv i elem ...)
       (non-opaque-bstruct-descriptor-identifier? #'<type>)
       (with-syntax (((((elem ...) val) ...)
                      (flatten-elems #'(elem ...))))
         #'(begin
             ((<type> setter elem ...) bv i val)
             ...))))))
(define-syntax bstruct-unpack
  (lambda (stx)
    (syntax-case stx ()
      ((_ <type> bv i elem ...)
       (non-opaque-bstruct-descriptor-identifier? #'<type>)
       #`(values
          #,@(map (lambda (elem)
                    (syntax-case elem ()
                      ((e ...)
                       #'((<type> getter e ...) bv i))
                      (e
                       #'((<type> getter e) bv i))))
                  #'(elem ...)))))))

;; Allocate a fresh bstruct that wraps a fresh bytevector big
;; enough to store the entire structure.
(define-syntax bstruct-alloc
  (syntax-rules ()
    ((_ (<type> n) (i elem ...) ...)
     (let* ((size (bstruct-sizeof <type>))
            (len (* size n))
            (bv (make-bytevector len 0)))
       (bstruct-pack! <type> bv (* size i) elem ...)
       ...
       (bstruct-wrap <type> bv 0)))
    ((_ <type> elem ...)
     (bstruct-alloc (<type> 1) (0 elem ...)))))

;; Return the value of some elements.
(define-syntax bstruct-ref
  (syntax-rules ()
    ((_ (<type> i) bs elem ...)         ; array
     (match bs
       (($ <type> bv (? u64? offset) (? u64? n))
        (assert (u64? i) 'bstruct-ref)
        (check-size i n 'bstruct-ref)
        (let ((offset (+ offset (* (bstruct-sizeof <type>) i))))
          (bstruct-unpack <type> bv offset elem ...)))))
    ((_ <type> bs elem ...)
     (bstruct-ref (<type> 0) bs elem ...))))

;; Set the value of some elements.
(define-syntax bstruct-set!
  (syntax-rules ()
    ((_ (<type> i) bs elem ...)         ; array
     (match bs
       (($ <type> bv (? u64? offset) (? u64? n))
        (assert (u64? i) 'bstruct-set!)
        (check-size i n 'bstruct-set!)
        (let ((offset (+ offset (* (bstruct-sizeof <type>) i))))
          (bstruct-pack! <type> bv offset elem ...)))))
    ((_ <type> bs elem ...)
     (bstruct-set! (<type> 0) bs elem ...))))

;; Imperative/functional struct copying.
(define-syntax-rule (bstruct-copy! <type> src dst)
  (match src
    (($ <type> src-bv (? u64? src-offset) (? u64? src-n))
     (match dst
       (($ <type> dst-bv (? u64? dst-offset) (? u64? dst-n))
        (check-size src-n (1+ dst-n) 'bstruct-copy!)
        (bytevector-copy! src-bv src-offset
                          dst-bv dst-offset
                          (* (bstruct-sizeof <type>) src-n)))))))
(define-syntax-rule (bstruct-copy <type> src)
  (match src
    (($ <type> _ _ (? u64? n))
     (let ((dst (bstruct-alloc (<type> n))))
       (bstruct-copy! <type> src dst)
       dst))))

(define (identifier-eq? stx sym)
  (and (identifier? stx) (eq? (syntax->datum stx) sym)))

;; The big gnarly procedural macro!  Buckle up!
(define-syntax define-bstruct
  (lambda (stx)
    (define primitives
      '(u8 s8 u16 s16 u32 s32 u64 s64 f32 f64
           int uint long ulong short ushort
           size_t ssize_t ptrdiff_t intptr_t uintptr_t))
    (define (target-endianness? e)
      (eq? e (target-endianness)))
    (define (resolve-endianness e)
      (match e
        ('native (target-endianness))
        ('non-native (non-target-endianness))
        (_ e)))
    (define (identifier-memq? stx syms)
      (and (identifier? stx) (memq (syntax->datum stx) syms)))
    ;; Primitive getter/setter helpers
    (define (ref/endianness proc endianness)
      #`(lambda (bv i)
          (#,proc bv i #,endianness)))
    (define (set!/endianness proc endianness)
      #`(lambda (bv i x)
          (#,proc bv i x #,endianness)))
    (define (uint-ref size endianness)
      #`(lambda (bv i)
          (bytevector-uint-ref bv i '#,(datum->syntax #f endianness) #,size)))
    (define (uint-set! size endianness)
      #`(lambda (bv i x)
          (bytevector-uint-set! bv i x '#,(datum->syntax #f endianness) #,size)))
    (define (sint-ref size endianness)
      #`(lambda (bv i)
          (bytevector-sint-ref bv i '#,(datum->syntax #f endianness) #,size)))
    (define (sint-set! size endianness)
      #`(lambda (bv i x)
          (bytevector-sint-set! bv i x '#,(datum->syntax #f endianness) #,size)))
    ;; Scalar types are divided into two categories: machine indepenent
    ;; and machine dependent.  The machine independent types (i32, f32,
    ;; etc.) have a known size and alignment on all platforms.  The
    ;; machine dependent types have a size and alignment that can vary
    ;; depending on the ABI of the system that is compiling the code.
    (define (abi-ref type e) ; e for endianness
      (match type
        ('uint (uint-ref (sizeof* 'uint) e))
        ('int (sint-ref (sizeof* 'int) e))
        ('ulong (uint-ref (sizeof* 'ulong) e))
        ('long (sint-ref (sizeof* 'long) e))
        ('ushort (uint-ref (sizeof* 'ushort) e))
        ('short (sint-ref (sizeof* 'short) e))
        ('size_t (uint-ref (sizeof* 'size_t) e))
        ('ssize_t (sint-ref (sizeof* 'ssize_t) e))
        ('ptrdiff_t (sint-ref (sizeof* 'ptrdiff_t) e))
        ('intptr_t (sint-ref (sizeof* 'intptr_t) e))
        ('uintptr_t (uint-ref (sizeof* 'uintptr_t) e))))
    (define (abi-set! type e)
      (match type
        ('uint (uint-set! (sizeof* 'uint) e))
        ('int (sint-set! (sizeof* 'int) e))
        ('ulong (uint-set! (sizeof* 'ulong) e))
        ('long (sint-set! (sizeof* 'long) e))
        ('ushort (uint-set! (sizeof* 'ushort) e))
        ('short (sint-set! (sizeof* 'short) e))
        ('size_t (uint-set! (sizeof* 'size_t) e))
        ('ssize_t (sint-set! (sizeof* 'ssize_t) e))
        ('ptrdiff_t (sint-set! (sizeof* 'ptrdiff_t) e))
        ('intptr_t (sint-set! (sizeof* 'intptr_t) e))
        ('uintptr_t (uint-set! (sizeof* 'uintptr_t) e))))
    (define (primitive-getter size native? type)
      (if native?
          (match type
            ('u8 #'bytevector-u8-ref)
            ('s8 #'bytevector-s8-ref)
            ('u16 #'bytevector-u16-native-ref)
            ('s16 #'bytevector-s16-native-ref)
            ('u32 #'bytevector-u32-native-ref)
            ('s32 #'bytevector-s32-native-ref)
            ('u64 #'bytevector-u64-native-ref)
            ('s64 #'bytevector-s64-native-ref)
            ('f32 #'bytevector-ieee-single-native-ref)
            ('f64 #'bytevector-ieee-double-native-ref)
            (_ (abi-ref type (target-endianness))))
          (let ((e (non-target-endianness)))
            (match type
              ('u8 #'bytevector-u8-ref)
              ('s8 #'bytevector-s8-ref)
              ('u16 (ref/endianness #'bytevector-u16-ref e))
              ('s16 (ref/endianness #'bytevector-s16-ref e))
              ('u32 (ref/endianness #'bytevector-u32-ref e))
              ('s32 (ref/endianness #'bytevector-s32-ref e))
              ('u64 (ref/endianness #'bytevector-u64-ref e))
              ('s64 (ref/endianness #'bytevector-s64-ref e))
              ('f32 (ref/endianness #'bytevector-ieee-single-ref e))
              ('f64 (ref/endianness #'bytevector-ieee-double-ref e))
              (_ (abi-ref type e))))))
    (define (primitive-setter size native? type)
      (if native?
          (match type
            ('u8 #'bytevector-u8-set!)
            ('s8 #'bytevector-s8-set!)
            ('u16 #'bytevector-u16-native-set!)
            ('s16 #'bytevector-s16-native-set!)
            ('u32 #'bytevector-u32-native-set!)
            ('s32 #'bytevector-s32-native-set!)
            ('u64 #'bytevector-u64-native-set!)
            ('s64 #'bytevector-s64-native-set!)
            ('f32 #'bytevector-ieee-single-native-set!)
            ('f64 #'bytevector-ieee-double-native-set!)
            (_ (abi-set! type (target-endianness))))
          (let ((e (non-target-endianness)))
            (match type
              ('u8 #'bytevector-u8-set!)
              ('s8 #'bytevector-s8-set!)
              ('u16 (set!/endianness #'bytevector-u16-set! e))
              ('s16 (set!/endianness #'bytevector-s16-set! e))
              ('u32 (set!/endianness #'bytevector-u32-set! e))
              ('s32 (set!/endianness #'bytevector-s32-set! e))
              ('u64 (set!/endianness #'bytevector-u64-set! e))
              ('s64 (set!/endianness #'bytevector-s64-set! e))
              ('f32 (set!/endianness #'bytevector-ieee-single-set! e))
              ('f64 (set!/endianness #'bytevector-ieee-double-set! e))
              (_ (abi-set! type e))))))
    ;; Types can be recursive by referencing a type name within the
    ;; same type group in a pointer expression.
    ;;
    ;; For example:
    ;;
    ;; (define-bstruct linked-list
    ;;   (struct (item int) (next (* linked-list))))
    ;;
    ;; To make this work, we keep a side table mapping type names to
    ;; pointer type accessor forms that need to be patched with a
    ;; cyclical reference *after* all the types are defined.
    (define recursive-pointers (make-hash-table))
    (define (sizeof type)
      (match type
        ((or ('scalar size _ _ _)
             ('struct size _ _)
             ('union size _ _)
             ('array size _ _ _)
             ('pointer size _ _))
         size)
        (('opaque) 0)
        ((? bstruct-descriptor-identifier?)
         (bstruct-descriptor-identifier-size type))))
    (define (alignof type)
      (match type
        ((or ('scalar _ align _ _)
             ('struct _ align _)
             ('union _ align _)
             ('array _ align _ _)
             ('pointer _ align _))
         align)
        (('opaque) 0)
        ((? bstruct-descriptor-identifier?)
         (bstruct-descriptor-identifier-alignment type))))
    (define (compute-type expr accessor type-ids packed? endianness)
      (syntax-case expr ()
        ;; Modifiers
        ((packed expr)
         (identifier-eq? #'packed 'packed)
         (compute-type #'expr accessor type-ids #t endianness))
        ((unpacked expr)
         (identifier-eq? #'unpacked 'unpacked)
         (compute-type #'expr accessor type-ids #f endianness))
        ((endian e expr)
         (and (identifier-eq? #'endian 'endian)
              (identifier-memq? #'e '(native non-native big little)))
         (compute-type #'expr accessor type-ids packed?
                       (resolve-endianness (syntax->datum #'e))))
        ;; Previously defined types.  'packed?' and 'endianness' do
        ;; not apply here.
        (type-id
         (bstruct-descriptor-identifier? #'type-id)
         #'type-id)
        ;; Primitive scalars
        (primitive
         (identifier-memq? #'primitive primitives)
         (let ((type (syntax->datum #'primitive)))
           `(scalar ,(sizeof* type)
                    ,(alignof* type)
                    ,(target-endianness? endianness)
                    ,type)))
        ;; Structs
        ((struct (field-name field-expr) ...)
         (and (identifier-eq? #'struct 'struct)
              (every identifier? #'(field-name ...)))
         (let loop ((field-exprs #'((field-name field-expr) ...))
                    (fields '()) (offset 0) (alignment 0))
           (syntax-case field-exprs ()
             (()
              ;; Round up to a multiple of alignment to get final
              ;; size.
              (let ((size (* (ceiling (/ offset alignment)) alignment)))
                `(struct ,size ,alignment ,(reverse fields))))
             ;; An underscore indicates a pseudo-field that is just
             ;; for padding.  It is not included in the struct field
             ;; list and just adds to the offset.
             (((underscore expr) . rest-exprs)
              (identifier-eq? #'underscore '_)
              (let ((type (compute-type #'expr #f type-ids packed? endianness)))
                (loop #'rest-exprs fields (+ offset (sizeof type)) alignment)))
             (((name expr) . rest-exprs)
              (let* ((type (compute-type #'expr
                                         #`(struct-field-ref #,accessor 'name)
                                         type-ids packed? endianness))
                     (field-alignment (alignof type))
                     (padding (if packed?
                                  0
                                  (modulo (- field-alignment
                                             (modulo offset field-alignment))
                                          field-alignment)))
                     (offset (+ offset padding))
                     (alignment (max alignment field-alignment)))
                (loop #'rest-exprs
                      (cons (list (syntax->datum #'name) offset type) fields)
                      (+ offset (sizeof type))
                      alignment))))))
        ;; Unions
        ((union (field-name field-expr) ...)
         (and (identifier-eq? #'union 'union)
              (every identifier? #'(field-name ...)))
         (let loop ((field-exprs #'((field-name field-expr) ...))
                    (fields '()) (size 0) (alignment 0))
           (syntax-case field-exprs ()
             (()
              `(union ,size ,alignment ,(reverse fields)))
             (((underscore expr) . rest-exprs)
              (identifier-eq? #'underscore '_)
              (let ((type (compute-type #'expr #f type-ids packed? endianness)))
                (loop #'rest-exprs fields (max size (sizeof type)) alignment)))
             (((name expr) . rest-exprs)
              (let ((type (compute-type #'expr
                                        #`(struct-field-ref #,accessor 'name)
                                        type-ids packed? endianness)))
                (loop #'rest-exprs
                      (cons (list (syntax->datum #'name) type) fields)
                      (max size (sizeof type))
                      (max alignment (alignof type))))))))
        ;; Arrays
        ((array length expr)
         (and (identifier-eq? #'array 'array)
              (exact-integer? (syntax->datum #'length)))
         (let ((length (syntax->datum #'length))
               (type (compute-type #'expr #`(array-type #,accessor)
                                   type-ids packed? endianness)))
           `(array ,(* (sizeof type) length) ,(alignof type) ,length ,type)))
        ;; Pointers
        ((pointer expr)
         (identifier-eq? #'pointer '*)
         (let ((size (ffi:sizeof '*))
               (align (ffi:alignof '*)))
           (let loop ((expr #'expr))
             (syntax-case expr ()
               ;; Void pointer
               (void
                (identifier-eq? #'void 'void)
                `(pointer ,size ,align void))
               ;; Primitive pointer
               (prim
                (identifier-memq? #'prim primitives)
                `(pointer ,size ,align
                          ,(compute-type #'prim #f type-ids packed? endianness)))
               ;; Pointer to a pointer
               ((pointer expr)
                (identifier-eq? #'pointer '*)
                `(pointer ,size ,align ,(loop #'expr)))
               ;; Recursive reference to a type within this type group.
               (type-id
                (any (lambda (id) (bound-identifier=? #'type-id id)) type-ids)
                (let ((pointer `(pointer ,size ,align (recur ,#'type-id))))
                  ;; Make a note that the recursive reference needs to be
                  ;; made via mutation after all types in the group are
                  ;; defined.
                  (hashq-set! recursive-pointers
                              (syntax->datum #'type-id)
                              (cons accessor
                                    (hashq-ref recursive-pointers
                                               (syntax->datum #'type-id)
                                               '())))
                  pointer))
               ;; Reference to a type outside of this type group.
               (type-id
                (bstruct-descriptor-identifier? #'type-id)
                `(pointer ,size ,align ,#'type-id))))))
        ;; Opaque types
        (opaque
         (identifier-eq? #'opaque 'opaque)
         '(opaque))))
    (define (type->syntax type)
      (match type
        ((? identifier? type)
         type)
        (('scalar size alignment native? type)
         #`(make-scalar #,size #,alignment #,native? '#,(datum->syntax #f type)))
        (('struct size alignment fields)
         (let ((fields* (map (match-lambda
                               ((name offset type)
                                #`(make-struct-field '#,(datum->syntax #f name)
                                                     #,offset
                                                     #,(type->syntax type))))
                             fields)))
           #`(make-struct #,size #,alignment (list #,@fields*))))
        (('union size alignment fields)
         (let ((fields* (map (match-lambda
                               ((name type)
                                #`(make-union-field '#,(datum->syntax #f name)
                                                    #,(type->syntax type))))
                             fields)))
           #`(make-union #,size #,alignment (list #,@fields*))))
        (('array size alignment length type)
         #`(make-array #,size #,alignment #,length #,(type->syntax type)))
        (('pointer size alignment ('recur type))
         #`(make-pointer #,size #,alignment #f))
        (('pointer size alignment 'void)
         #`(make-pointer #,size #,alignment #f))
        (('pointer size alignment type)
         #`(make-pointer #,size #,alignment #,(type->syntax type)))
        (('opaque)
         #'(make-opaque))))
    (define (expand-accessor proc stx id op type group)
      #`(syntax-case #,stx ()
          (() #'#,(proc id 0))          ; self reference
          ((elem :::)
           #,(let loop ((stx #'#'(elem :::)) (type type) (offset 0))
               (match type
                 ((? bstruct-descriptor-identifier? type)
                  ;; Recursively invoke macro for referenced type to produce
                  ;; the accessor.
                  #`(syntax-case #,stx ()
                      ((elem :::)
                       #'#,(proc #`(#,type #,(datum->syntax #f op) elem :::)
                                 offset))))
                 (('scalar _ _ _ _)
                  #`(syntax-case #,stx ()
                      (() #'#,(proc type offset))))
                 (('struct _ _ fields)
                  #`(syntax-case #,stx ()
                      ((e elem :::)
                       (match (syntax->datum #'e)
                         #,@(map (match-lambda
                                   ((name offset* type)
                                    #`('#,(datum->syntax #f name)
                                       #,(loop #'#'(elem :::) type
                                               #`(+ #,offset #,offset*)))))
                                 fields)
                         (_ #'(error "no such struct field" 'e))))))
                 (('union _ _ fields)
                  #`(syntax-case #,stx ()
                      ((e elem :::)
                       (match (syntax->datum #'e)
                         #,@(map (match-lambda
                                   ((name type)
                                    #`('#,(datum->syntax #f name)
                                       #,(loop #'#'(elem :::) type offset))))
                                 fields)
                         (_ #'(error "no such union field" 'e))))))
                 (('array _ _ length type)
                  ;; Need to generate a unique name here to capture
                  ;; the 'e' containing the array index expression as
                  ;; 'e' could get shadowed later.
                  (with-syntax (((i) (generate-temporaries '(i))))
                    #`(syntax-case #,stx ()
                        ((e elem :::)
                         (with-syntax ((i #'e))
                           #,(loop #'#'(elem :::)
                                   type
                                   #`(+ #,offset
                                        (* (let ()
                                             ;; if 'i' is a constant then
                                             ;; these checks will be elided
                                             ;; by the compiler.
                                             (assert (u64? i) 'bstruct-accessor)
                                             (assert (< -1 i #,length)
                                                     'bstruct-accessor)
                                             i)
                                           #,(sizeof type)))))))))
                 ;; Void pointers can only be referenced, not
                 ;; dereferenced.
                 (('pointer _ _ 'void)
                  #`(syntax-case #,stx ()
                      (() #'#,(proc type offset))))
                 (('pointer _ _ type*)
                  (call-with-values (lambda ()
                                      (match type*
                                        (('recur type*)
                                         (values type* #t))
                                        (type*
                                         (values type* #f))))
                    (lambda (type* recur?)
                      #`(let ()
                          (define (expand-pointer-accessor stx)
                            (syntax-case stx ()
                              ;; Pointer reference
                              (() #'#,(proc type offset))
                              ;; Pointer dereference.  Sigh, it's
                              ;; complicated...
                              (((* index) elem :::)
                               (identifier-eq? #'* '*)
                               (let ((offset #,offset)
                                     (e '#,(datum->syntax #f (target-endianness)))
                                     (ptr-size #,(target-word-size))
                                     ;; For recursive types, we don't
                                     ;; yet have a defined macro to
                                     ;; query for the size, so we have
                                     ;; to look it up in the group
                                     ;; alist.
                                     (size #,(sizeof (if recur?
                                                         (assoc-ref group type*)
                                                         type*)))
                                     (body #,(match type*
                                               ((? syntax?)
                                                #`#'#,(proc #`(#,type*
                                                               #,(datum->syntax #f op)
                                                               elem :::)
                                                            0))
                                               (_
                                                (loop #'#'(elem :::) type* 0)))))
                                 ;; 'bv' and 'i' are the lexical
                                 ;; variables containing the
                                 ;; bytevector and offset for
                                 ;; getting/setting.  Every time we
                                 ;; encounter a pointer dereference we
                                 ;; need to shadow the old variables
                                 ;; with new ones.
                                 #`(let* ((e '#,(datum->syntax #f e))
                                          (base (bytevector-sint-ref bv (+ i #,offset)
                                                                     e #,ptr-size))
                                          (address (+ base (* #,size index)))
                                          (ptr (ffi:make-pointer address))
                                          (bv (ffi:pointer->bytevector ptr #,size))
                                          (i 0))
                                     #,body)))
                              ;; Pointer dereference with implied
                              ;; index of 0.
                              ((* elem :::)
                               (identifier-eq? #'* '*)
                               (expand-pointer-accessor #'((* 0) elem :::)))))
                          (expand-pointer-accessor #,stx)))))
                 (('opaque)
                  #'(error "cannot access opaque type")))))))
    (define (macro-for-type id type-id type group)
      (define (self-identifier? other)
        (eq? id other))
      (let ((self-size (sizeof type)))
        #`(define-syntax #,id
            (with-ellipsis :::
              (lambda (stx)
                ;; Since syntax transformers are procedures, we can
                ;; stash useful information in procedure properties that
                ;; 'define-bstruct' can use if/when a bstruct type
                ;; is referenced within another type definition.
                #((bstruct? . #t)
                  (bstruct-opaque? . #,(eq? type '(opaque)))
                  (bstruct-size . #,(sizeof type))
                  (bstruct-alignment . #,(alignof type)))
                (syntax-case stx ()
                  ;; Identifier syntax to provide the illusion that this
                  ;; macro is just an ordinary variable.
                  (self
                   (identifier? #'self)
                   #'#,type-id)
                  ;; Private interface for code generation.
                  ((_ offset elem :::)
                   (identifier-eq? #'offset 'offset)
                   #`(lambda (bv i)
                       #,#,(expand-accessor
                            (lambda (type offset)
                              (match type
                                ((and sub-offset ((? syntax?) ...))
                                 #`(#,sub-offset bv (+ i #,offset)))
                                (_ #`(values bv #,offset))))
                            #'#'(elem :::) id 'offset type group)))
                  ((_ getter elem :::)
                   (identifier-eq? #'getter 'getter)
                   #`(lambda (bv i)
                       #,#,(expand-accessor
                            (lambda (type offset)
                              (match type
                                ((or (? self-identifier?)
                                     (? bstruct-descriptor-identifier?))
                                 #`(make-bstruct #,type bv (+ i #,offset)))
                                ((and sub-get ((? syntax?) ...))
                                 #`(#,sub-get bv (+ i #,offset)))
                                (('pointer _ _ 'void)
                                 #`(ffi:make-pointer
                                    (bytevector-sint-ref bv (+ i #,offset)
                                                         '#,(datum->syntax
                                                             #f (target-endianness))
                                                         #,(target-word-size))))
                                (('pointer size _ (or ('recur type) type))
                                 #`(ffi:make-pointer
                                    (bytevector-sint-ref bv (+ i #,offset)
                                                         '#,(datum->syntax
                                                             #f (target-endianness))
                                                         #,(target-word-size))))
                                (('scalar size align native? type)
                                 #`(#,(primitive-getter size native? type)
                                    bv (+ i #,offset)))))
                            #'#'(elem :::) id 'getter type group)))
                  ((_ setter elem :::)
                   (identifier-eq? #'setter 'setter)
                   #`(lambda (bv i x)
                       #,#,(expand-accessor
                            (lambda (type offset)
                              (match type
                                ((? self-identifier?)
                                 #`(match x
                                     (($ #,type src j)
                                      (bytevector-copy! src j bv
                                                        (+ i #,offset)
                                                        #,self-size))))
                                ((? bstruct-descriptor-identifier?)
                                 #`(match x
                                     (($ #,type src j)
                                      (bytevector-copy! src j bv
                                                        (+ i #,offset)
                                                        #,(sizeof type)))))
                                ((and sub-set! ((? syntax?) ...))
                                 #`(#,sub-set! bv (+ i #,offset) x))
                                (('pointer size _ 'void)
                                 #`(bytevector-sint-set! bv (+ i #,offset)
                                                         (ffi:pointer-address x)
                                                         '#(datum->syntax
                                                            #f (target-endianness))
                                                         #,(target-word-size)))
                                (('pointer size _ (or ('recur type) type))
                                 #`(bytevector-sint-set! bv (+ i #,offset)
                                                         (ffi:pointer-address x)
                                                         '#(datum->syntax
                                                            #f (target-endianness))
                                                         #,(target-word-size)))
                                (('scalar size align native? type)
                                 (let ((setter (primitive-setter size native? type)))
                                   #`(#,setter bv (+ i #,offset) x)))))
                            #'#'(elem :::) id 'setter type group)))))))))
    (syntax-case stx ()
      ;; Type group definition.  Types defined in the same group can
      ;; contain recursive pointer references to each other.
      ((_ (id expr . kwargs) ...)
       (not (null? #'(id ...)))
       (with-syntax (((type-id ...)
                      (map (lambda (stx)
                             ;; This ID is non-hygienic and also *not*
                             ;; gensym'd so it's the same name every
                             ;; time for predictable compilation,
                             ;; especially in the presence of 'make
                             ;; -j' parallelism.
                             ;;
                             ;; Is there a better way to do this?
                             ;; Probably.
                             (let ((type-id (symbol-append '%%bstruct-record-type-
                                                           (syntax->datum stx))))
                               (datum->syntax #f type-id)))
                           #'(id ...))))
         (let* ((ids #'(id ...))
                (type-ids #'(type-id ...))
                (kwargs #'(kwargs ...))
                (types (map (lambda (id expr)
                              (compute-type expr
                                            #`(bstruct-descriptor-type #,id)
                                            ids #f (target-endianness)))
                            #'(id ...)
                            #'(expr ...)))
                (group (map cons #'(id ...) types)))
           #`(begin
               ;; First, define the descriptors using gensym'd names.
               #,@(map (lambda (id temp-id kwargs type)
                         #`(define #,temp-id
                             (make-bstruct-descriptor '#,id
                                                      #,(type->syntax type)
                                                      #,@kwargs)))
                       ids type-ids kwargs types)
               ;; Then tie the knot for recursive pointer types.
               #,@(append-map (lambda (id)
                                (map (lambda (accessor)
                                       #`(set-pointer-type! #,accessor #,id))
                                     (hashq-ref recursive-pointers
                                                (syntax->datum id) '())))
                              ids)
               ;; Finally, define macros using the true names that
               ;; wrap the gensym'd variables.
               #,@(map (lambda (id type-id type)
                         (macro-for-type id type-id type group))
                       ids type-ids types)))))
      ;; A single type definition is a type group of one.
      ((_ id expr . kwargs)
       #'(define-bstruct (id expr . kwargs))))))