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+;;;; json.scm --- JSON reader/writer
+;;;; Copyright © 2015, 2017 David Thompson <davet@gnu.org>
+;;;; Copyright © 2017 Christopher Allan Webber <cwebber@dustycloud.org>
+;;;;
+;;;; This library is free software; you can redistribute it and/or
+;;;; modify it under the terms of the GNU Lesser General Public
+;;;; License as published by the Free Software Foundation; either
+;;;; version 3 of the License, or (at your option) any later version.
+;;;;
+;;;; This library is distributed in the hope that it will be useful,
+;;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+;;;; Lesser General Public License for more details.
+;;;;
+;;;; You should have received a copy of the GNU Lesser General Public
+;;;; License along with this library; if not, write to the Free Software
+;;;; Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+;;;; 02110-1301 USA
+
+(define-module (aws json)
+  #:use-module (ice-9 match)
+  #:export (read-json write-json))
+
+(define (json-error port)
+  (throw 'json-error port))
+
+(define (assert-char port char)
+  "Read a character from PORT and throw an invalid JSON error if the
+character is not CHAR."
+  (unless (eqv? (read-char port) char)
+    (json-error port)))
+
+(define (whitespace? char)
+  "Return #t if CHAR is a whitespace character."
+  (char-set-contains? char-set:whitespace char))
+
+(define (consume-whitespace port)
+  "Discard characters from PORT until a non-whitespace character is
+encountered.."
+  (match (peek-char port)
+    ((? eof-object?) *unspecified*)
+    ((? whitespace?)
+     (read-char port)
+     (consume-whitespace port))
+    (_ *unspecified*)))
+
+(define (make-keyword-reader keyword value)
+  "Parse the keyword symbol KEYWORD as VALUE."
+  (let ((str (symbol->string keyword)))
+    (lambda (port)
+      (let loop ((i 0))
+        (cond
+         ((= i (string-length str)) value)
+         ((eqv? (string-ref str i) (read-char port))
+          (loop (1+ i)))
+         (else (json-error port)))))))
+
+(define read-true (make-keyword-reader 'true #t))
+(define read-false (make-keyword-reader 'false #f))
+(define read-null (make-keyword-reader 'null 'null))
+
+(define (read-hex-digit port)
+  "Read a hexadecimal digit from PORT."
+  (match (read-char port)
+    (#\0 0)
+    (#\1 1)
+    (#\2 2)
+    (#\3 3)
+    (#\4 4)
+    (#\5 5)
+    (#\6 6)
+    (#\7 7)
+    (#\8 8)
+    (#\9 9)
+    ((or #\A #\a) 10)
+    ((or #\B #\b) 11)
+    ((or #\C #\c) 12)
+    ((or #\D #\d) 13)
+    ((or #\E #\e) 14)
+    ((or #\F #\f) 15)
+    (_ (json-error port))))
+
+(define (read-utf16-character port)
+  "Read a hexadecimal encoded UTF-16 character from PORT."
+  (integer->char
+   (+ (* (read-hex-digit port) (expt 16 3))
+      (* (read-hex-digit port) (expt 16 2))
+      (* (read-hex-digit port) 16)
+      (read-hex-digit port))))
+
+(define (read-escape-character port)
+  "Read escape character from PORT."
+  (match (read-char port)
+    (#\" #\")
+    (#\\ #\\)
+    (#\/ #\/)
+    (#\b #\backspace)
+    (#\f #\page)
+    (#\n #\newline)
+    (#\r #\return)
+    (#\t #\tab)
+    (#\u (read-utf16-character port))
+    (_ (json-error port))))
+
+(define (read-string port)
+  "Read a JSON encoded string from PORT."
+  (assert-char port #\")
+  (let loop ((result '()))
+    (match (read-char port)
+      ((? eof-object?) (json-error port))
+      (#\" (list->string (reverse result)))
+      (#\\ (loop (cons (read-escape-character port) result)))
+      (char (loop (cons char result))))))
+
+(define char-set:json-digit
+  (char-set #\0 #\1 #\2 #\3 #\4 #\5 #\6 #\7 #\8 #\9))
+
+(define (digit? char)
+  (char-set-contains? char-set:json-digit char))
+
+(define (read-digit port)
+  "Read a digit 0-9 from PORT."
+  (match (read-char port)
+    (#\0 0)
+    (#\1 1)
+    (#\2 2)
+    (#\3 3)
+    (#\4 4)
+    (#\5 5)
+    (#\6 6)
+    (#\7 7)
+    (#\8 8)
+    (#\9 9)
+    (else (json-error port))))
+
+(define (read-digits port)
+  "Read a sequence of digits from PORT."
+  (let loop ((result '()))
+    (match (peek-char port)
+      ((? eof-object?)
+       (reverse result))
+      ((? digit?)
+       (loop (cons (read-digit port) result)))
+      (else (reverse result)))))
+
+(define (list->integer digits)
+  "Convert the list DIGITS to an integer."
+  (let loop ((i (1- (length digits)))
+             (result 0)
+             (digits digits))
+    (match digits
+      (() result)
+      ((n . tail)
+       (loop (1- i)
+             (+ result (* n (expt 10 i)))
+             tail)))))
+
+(define (read-positive-integer port)
+  "Read a positive integer with no leading zeroes from PORT."
+  (match (read-digits port)
+    ((0 . _)
+     (json-error port)) ; no leading zeroes allowed
+    ((digits ...)
+     (list->integer digits))))
+
+(define (read-exponent port)
+  "Read exponent from PORT."
+  (define (read-expt)
+    (list->integer (read-digits port)))
+
+  (unless (memv (read-char port) '(#\e #\E))
+    (json-error port))
+
+  (match (peek-char port)
+    ((? eof-object?)
+     (json-error port))
+    (#\-
+     (read-char port)
+     (- (read-expt)))
+    (#\+
+     (read-char port)
+     (read-expt))
+    ((? digit?)
+     (read-expt))
+    (_ (json-error port))))
+
+(define (read-fraction port)
+  "Read fractional number part from PORT as an inexact number."
+  (let* ((digits      (read-digits port))
+         (numerator   (list->integer digits))
+         (denomenator (expt 10 (length digits))))
+    (/ numerator denomenator)))
+
+(define (read-positive-number port)
+  "Read a positive number from PORT."
+  (let* ((integer (match (peek-char port)
+                    ((? eof-object?)
+                     (json-error port))
+                    (#\0
+                     (read-char port)
+                     0)
+                    ((? digit?)
+                     (read-positive-integer port))
+                    (_ (json-error port))))
+         (fraction (match (peek-char port)
+                     (#\.
+                      (read-char port)
+                      (read-fraction port))
+                     (_ 0)))
+         (exponent (match (peek-char port)
+                     ((or #\e #\E)
+                      (read-exponent port))
+                     (_ 0)))
+         (n (* (+ integer fraction) (expt 10 exponent))))
+
+    ;; Keep integers as exact numbers, but convert numbers encoded as
+    ;; floating point numbers to an inexact representation.
+    (if (zero? fraction)
+        n
+        (exact->inexact n))))
+
+(define (read-number port)
+  "Read a number from PORT"
+  (match (peek-char port)
+    ((? eof-object?)
+     (json-error port))
+    (#\-
+     (read-char port)
+     (- (read-positive-number port)))
+    ((? digit?)
+     (read-positive-number port))
+    (_ (json-error port))))
+
+(define (read-object port)
+  "Read key/value map from PORT."
+  (define (read-key+value-pair)
+    (let ((key (read-string port)))
+      (consume-whitespace port)
+      (assert-char port #\:)
+      (consume-whitespace port)
+      (let ((value (read-value port)))
+        (cons key value))))
+
+  (assert-char port #\{)
+  (consume-whitespace port)
+
+  (if (eqv? #\} (peek-char port))
+      (begin
+        (read-char port)
+        '()) ; empty object
+      (cons (read-key+value-pair)
+            (let loop ()
+              (consume-whitespace port)
+              (match (peek-char port)
+                (#\, ; read another value
+                 (read-char port)
+                 (consume-whitespace port)
+                 (cons (read-key+value-pair) (loop)))
+                (#\} ; end of object
+                 (read-char port)
+                 '())
+                (_ (json-error port)))))))
+
+(define (read-array port)
+  "Read array from PORT."
+  (assert-char port #\[)
+  (consume-whitespace port)
+
+  (list->vector
+   (if (eqv? #\] (peek-char port))
+       (begin
+         (read-char port)
+         '() ); empty array
+       (cons (read-value port)
+             (let loop ()
+               (consume-whitespace port)
+               (match (peek-char port)
+                 (#\, ; read another value
+                  (read-char port)
+                  (consume-whitespace port)
+                  (cons (read-value port) (loop)))
+                 (#\] ; end of array
+                  (read-char port)
+                  '())
+                 (_ (json-error port))))))))
+
+(define (read-value port)
+  "Read a JSON value from PORT."
+  (consume-whitespace port)
+  (match (peek-char port)
+    ((? eof-object?) (json-error port))
+    (#\" (read-string port))
+    (#\{ (read-object port))
+    (#\[ (read-array port))
+    (#\t (read-true port))
+    (#\f (read-false port))
+    (#\n (read-null port))
+    ((or #\- (? digit?))
+     (read-number port))
+    (_ (json-error port))))
+
+(define (read-json port)
+  "Read JSON text from port and return an s-expression representation."
+  (let ((result (read-value port)))
+    (consume-whitespace port)
+    (unless (eof-object? (peek-char port))
+      (json-error port))
+    result))
+
+
+;;;
+;;; Writer
+;;;
+
+(define (write-string str port)
+  "Write STR to PORT in JSON string format."
+  (define (escape-char char)
+    (display (match char
+               (#\" "\\\"")
+               (#\\ "\\\\")
+               (#\/ "\\/")
+               (#\backspace "\\b")
+               (#\page "\\f")
+               (#\newline "\\n")
+               (#\return "\\r")
+               (#\tab "\\t")
+               (_ char))
+             port))
+
+  (display "\"" port)
+  (string-for-each escape-char str)
+  (display "\"" port))
+
+(define (write-object object port)
+  "Write ALIST to PORT in JSON object format."
+  ;; Keys may be strings or symbols.
+  (define key->string
+    (match-lambda
+     ((? string? key) key)
+     ((? symbol? key) (symbol->string key))))
+
+  (define (write-kv-pair key value)
+    (write-string (key->string key) port)
+    (display ":" port)
+    (write-json value port))
+
+  (display "{" port)
+  (match object
+    (() #f)
+    ((front ... (end-key end-val))
+     (for-each (match-lambda
+                 ((key value)
+                  (write-kv-pair key value)
+                  (display "," port)))
+               front)
+     (write-kv-pair end-key end-val)))
+  (display "}" port))
+
+(define (write-array lst port)
+  "Write LST to PORT in JSON array format."
+  (display "[" port)
+  (match lst
+    (() #f)
+    ((front ... end)
+     (for-each (lambda (val)
+                 (write-json val port)
+                 (display "," port))
+               front)
+     (write-json end port)))
+  (display "]" port))
+
+(define (write-json exp port)
+  "Write EXP to PORT in JSON format."
+  (match exp
+    (#t (display "true" port))
+    (#f (display "false" port))
+    ('null (display "null" port))
+    ((? string? s) (write-string s port))
+    ((? real? n) (display n port))
+    (('@ . alist) (write-object alist port))
+    ((vals ...) (write-array vals port))))