Rework fz_streams.

Currently fz_streams have a 4K buffer within their header. The call
to read from a stream fills this buffer, resulting in more data being
pulled from any underlying stream than we might like. This causes
problems with the forthcoming 'leech' filter.

Here we simplify the fields available in the public stream header.
No specific buffer is given; simply the read and write pointers.

The underlying 'read' function is replaced by a 'next' function
that makes the next block of data available and returns the first
character of it (or EOF).

A caller to the 'next' function should supply the maximum number of
bytes that it knows it will need (possibly not now, but eventually).
This enables the underlying stream to efficiently decode just enough.
The underlying stream is free to return fewer, or a greater number
if it wants to.

The exact size of the 'block' of data returned will depend on the
filter in use and (possibly) the data therein.

Callers can get the currently available amount of data by calling
fz_available (but again should pass the maximum amount of data they know
they will need). The only time this will ever return 0 is if we have
hit EOF.

1 files changed, 18 insertions, 5 deletions

diff --git a/source/fitz/filter-lzw.c b/source/fitz/filter-lzw.c
index c78c8c3f..e13b0ef0 100644
--- a/source/fitz/filter-lzw.c
+++ b/source/fitz/filter-lzw.c
@@ -41,15 +41,18 @@ struct fz_lzwd_s
 
 	unsigned char bp[MAX_LENGTH];
 	unsigned char *rp, *wp;
+
+	unsigned char buffer[4096];
 };
 
 static int
-read_lzwd(fz_stream *stm, unsigned char *buf, int len)
+next_lzwd(fz_stream *stm, int len)
 {
 	fz_lzwd *lzw = stm->state;
 	lzw_code *table = lzw->table;
+	unsigned char *buf = lzw->buffer;
 	unsigned char *p = buf;
-	unsigned char *ep = buf + len;
+	unsigned char *ep;
 	unsigned char *s;
 	int codelen;
 
@@ -58,13 +61,17 @@ read_lzwd(fz_stream *stm, unsigned char *buf, int len)
 	int old_code = lzw->old_code;
 	int next_code = lzw->next_code;
 
+	if (len > sizeof(lzw->buffer))
+		len = sizeof(lzw->buffer);
+	ep = buf + len;
+
 	while (lzw->rp < lzw->wp && p < ep)
 		*p++ = *lzw->rp++;
 
 	while (p < ep)
 	{
 		if (lzw->eod)
-			return 0;
+			return EOF;
 
 		code = fz_read_bits(lzw->chain, code_bits);
 
@@ -162,7 +169,13 @@ read_lzwd(fz_stream *stm, unsigned char *buf, int len)
 	lzw->old_code = old_code;
 	lzw->next_code = next_code;
 
-	return p - buf;
+	stm->rp = buf;
+	stm->wp = p;
+	if (buf == p)
+		return EOF;
+	stm->pos += p - buf;
+
+	return *stm->rp++;
 }
 
 static void
@@ -228,5 +241,5 @@ fz_open_lzwd(fz_stream *chain, int early_change)
 		fz_rethrow(ctx);
 	}
 
-	return fz_new_stream(ctx, lzw, read_lzwd, close_lzwd, rebind_lzwd);
+	return fz_new_stream(ctx, lzw, next_lzwd, close_lzwd, rebind_lzwd);
 }