From 97476a28d8aa0c1403d4ba4815f8f07a7caa7b4e Mon Sep 17 00:00:00 2001
From: Kaz Kylheku <kaz@kylheku.com>
Date: Tue, 19 Apr 2016 19:02:18 -0700
Subject: Allow unlimited character pushback in unget-char.

Fixing read_until_match will require this feature.

* socket.c (dgram_get_char): Treat unget_c as a
cons-based stack; pop a character from it if
available.
(dgram_unget_char): Push the character onto unget_c
rather than storing the characer into unget_c.

* stream.c (stdio_get_char, stdio_unget_char): Closely
analogous changes to the ones in dgram_get_char
and dgram_unget_char, respectively.

* txr.1: Documentation improved and updated.
---
 socket.c | 15 ++++-----------
 stream.c | 15 +++++----------
 txr.1    | 17 ++++++++++++++++-
 3 files changed, 25 insertions(+), 22 deletions(-)

diff --git a/socket.c b/socket.c
index 7cb119ae..102127b9 100644
--- a/socket.c
+++ b/socket.c
@@ -434,10 +434,9 @@ static int dgram_get_byte_callback(mem_t *ctx)
 static val dgram_get_char(val stream)
 {
   struct dgram_stream *d = coerce(struct dgram_stream *, stream->co.handle);
-  val uc = d->unget_c;
-  if (uc) {
-    d->unget_c = nil;
-    return uc;
+
+  if (d->unget_c) {
+    return pop(&d->unget_c);
   } else {
     wint_t ch = utf8_decode(&d->ud, dgram_get_byte_callback,
                             coerce(mem_t *, d));
@@ -455,13 +454,7 @@ static val dgram_get_byte(val stream)
 static val dgram_unget_char(val stream, val ch)
 {
   struct dgram_stream *d = coerce(struct dgram_stream *, stream->co.handle);
-
-  if (d->unget_c){
-    d->err = EOVERFLOW;
-    uw_throwf(file_error_s, lit("unget-char overflow on ~a: "), stream, nao);
-  }
-
-  d->unget_c = ch;
+  mpush(ch, mkloc(d->unget_c, stream));
   return ch;
 }
 
diff --git a/stream.c b/stream.c
index 6b234e22..24dc79d2 100644
--- a/stream.c
+++ b/stream.c
@@ -683,11 +683,10 @@ val generic_get_line(val stream)
 static val stdio_get_char(val stream)
 {
   struct stdio_handle *h = coerce(struct stdio_handle *, stream->co.handle);
-  val uc = h->unget_c;
-  if (uc) {
-    h->unget_c = nil;
-    return uc;
-  }
+
+  if (h->unget_c)
+    return pop(&h->unget_c);
+
   if (h->f) {
     wint_t ch = utf8_decode(&h->ud, stdio_get_char_callback,
                             coerce(mem_t *, h->f));
@@ -709,11 +708,7 @@ static val stdio_get_byte(val stream)
 static val stdio_unget_char(val stream, val ch)
 {
   struct stdio_handle *h = coerce(struct stdio_handle *, stream->co.handle);
-
-  if (h->unget_c)
-    uw_throwf(file_error_s, lit("unget-char overflow on ~a: "), stream, nao);
-
-  h->unget_c = ch;
+  mpush(ch, mkloc(h->unget_c, stream));
   return ch;
 }
 
diff --git a/txr.1 b/txr.1
index 7a1d5b05..c07ca2c8 100644
--- a/txr.1
+++ b/txr.1
@@ -34230,7 +34230,18 @@ and likewise for
 and
 .codn unget-byte .
 
-Space is available for only one character or byte of pushback.
+Streams may require a pushed back byte or character to match
+the character which was previously read from that stream
+position, and may not allow a byte or character to be pushed
+back beyond the beginning of the stream.
+
+Space may be available for only one byte of pushback under the
+.code unget-byte
+operation.
+
+The number of characters that may be pushed back by
+.code unget-char
+is not limited.
 
 Pushing both a byte and a character, in either order, is also unsupported.
 Pushing a byte and then reading a character, or pushing a character and
@@ -34240,6 +34251,10 @@ If the stream is binary, then pushing back a byte decrements its position,
 except if the position is already zero. At that point, the position becomes
 indeterminate.
 
+The behavior of pushing back immediately after a
+.code seek-stream
+positioning operation is unspecified.
+
 .coNP Functions @, put-string @, put-line @ put-char and @ put-byte
 .synb
 .mets (put-string < string <> [ stream ])
-- 
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