Add offset support in peek-byte.

Add support for peek-byte offset up to 4 bytes, which is needed to
implement peek-char.

Author: dvanhorn

langs/outlaw/io.c

@@ -19,10 +19,19 @@ val_t read_byte(void)
 
 val_t peek_byte(void* fake_port, int offset)
 {
-  // offset is ignored for now
-  char c = getc(in);
+  char cs[3];
+  if ((offset < 0) || (offset > 3)) { exit(-1); }
+  int i;
+  char c;
+  for (i = 0; i < offset; i++) {
+    cs[i] = getc(in);
+  }
+  c = getc(in);
   ungetc(c, in);
-  return (c == EOF) ? val_wrap_eof() : val_wrap_int(c);
+  for (i = 0; i < offset; i++) {
+    ungetc(cs[offset-i-1], in);
+  }
+  return (c == EOF) ? val_wrap_eof() : val_wrap_int((unsigned char)c);
 }
 
 val_t write_byte(val_t c)

langs/outlaw/test/test-runner.rkt

@@ -752,6 +752,19 @@
                 (cons #\a ""))
   (check-equal? (run "ab" '(cons (read-char) (read-char)))
                 (cons '(#\a . #\b) ""))
+  (check-equal? (run "a" '(peek-byte (%current-input-port) 0))
+                (cons 97 ""))
+  (check-equal? (run "ab" '(cons (peek-byte (%current-input-port) 1) (read-byte)))
+                (cons (cons 98 97) ""))
+  (check-equal? (run "abc" '(cons (peek-byte (%current-input-port) 2)
+                                  (cons (read-byte) (read-byte))))
+                (cons (cons 99 (cons 97 98)) ""))
+  (check-equal? (run "a" '(peek-char))
+                (cons #\a ""))
+  (check-equal? (run "ab" '(cons (peek-char) (peek-char)))
+                (cons '(#\a . #\a) ""))
+  (check-equal? (run "λ" '(peek-char))
+                (cons #\λ ""))
   (check-equal? (run "" '(write-char #\a))
                 (cons (void) "a"))
   (check-equal? (run "" '(write-char #\newline))