parse: extract parse module

The `git_patch_parse_ctx` encapsulates both parser state as well as
options specific to patch parsing. To advance this state and keep it
consistent, we provide a few functions which handle advancing the
current position and accessing bytes of the patch contents. In fact,
these functions are quite generic and not related to patch-parsing by
themselves. Seeing that we have similar logic inside of other modules,
it becomes quite enticing to extract this functionality into its own
parser module.

To do so, we create a new module `parse` with a central struct called
`git_parse_ctx`. It encapsulates both the content that is to be parsed
as well as its lengths and the current position. `git_patch_parse_ctx`
now only contains this `parse_ctx` only, which is then accessed whenever
we need to touch the current parser. This is the first step towards
re-using this functionality across other modules which require parsing
functionality and remove code-duplication.

src/diff_parse.c

@@ -83,7 +83,7 @@ int git_diff_from_buffer(
 	ctx = git_patch_parse_ctx_init(content, content_len, NULL);
 	GITERR_CHECK_ALLOC(ctx);
 
-	while (ctx->remain_len) {
+	while (ctx->parse_ctx.remain_len) {
 		if ((error = git_patch_parse(&patch, ctx)) < 0)
 			break;
 

src/parse.c

+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+#include "parse.h"
+
+int git_parse_ctx_init(git_parse_ctx *ctx, const char *content, size_t content_len)
+{
+	if (content_len)
+		ctx->content = content;
+	else
+		ctx->content = NULL;
+
+	ctx->content_len = content_len;
+	ctx->remain = ctx->content;
+	ctx->remain_len = ctx->content_len;
+
+	return 0;
+}
+
+void git_parse_ctx_clear(git_parse_ctx *ctx)
+{
+	memset(ctx, 0, sizeof(*ctx));
+}
+
+void git_parse_advance_line(git_parse_ctx *ctx)
+{
+	ctx->line += ctx->line_len;
+	ctx->remain_len -= ctx->line_len;
+	ctx->line_len = git__linenlen(ctx->line, ctx->remain_len);
+	ctx->line_num++;
+}
+
+void git_parse_advance_chars(git_parse_ctx *ctx, size_t char_cnt)
+{
+	ctx->line += char_cnt;
+	ctx->remain_len -= char_cnt;
+	ctx->line_len -= char_cnt;
+}
+
+int git_parse_advance_expected(
+	git_parse_ctx *ctx,
+	const char *expected,
+	size_t expected_len)
+{
+	if (ctx->line_len < expected_len)
+		return -1;
+
+	if (memcmp(ctx->line, expected, expected_len) != 0)
+		return -1;
+
+	git_parse_advance_chars(ctx, expected_len);
+	return 0;
+}
+
+int git_parse_advance_ws(git_parse_ctx *ctx)
+{
+	int ret = -1;
+
+	while (ctx->line_len > 0 &&
+		ctx->line[0] != '\n' &&
+		git__isspace(ctx->line[0])) {
+		ctx->line++;
+		ctx->line_len--;
+		ctx->remain_len--;
+		ret = 0;
+	}
+
+	return ret;
+}
+
+int git_parse_advance_nl(git_parse_ctx *ctx)
+{
+	if (ctx->line_len != 1 || ctx->line[0] != '\n')
+		return -1;
+
+	git_parse_advance_line(ctx);
+	return 0;
+}

src/parse.h

+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+#include "common.h"
+
+typedef struct {
+	/* Original content buffer */
+	const char *content;
+	size_t content_len;
+
+	/* The remaining (unparsed) buffer */
+	const char *remain;
+	size_t remain_len;
+
+	const char *line;
+	size_t line_len;
+	size_t line_num;
+} git_parse_ctx;
+
+int git_parse_ctx_init(git_parse_ctx *ctx, const char *content, size_t content_len);
+void git_parse_ctx_clear(git_parse_ctx *ctx);
+
+#define git_parse_err(...) \
+	( giterr_set(GITERR_PATCH, __VA_ARGS__), -1 )
+
+#define git_parse_ctx_contains_s(ctx, str) \
+	git_parse_ctx_contains(ctx, str, sizeof(str) - 1)
+
+GIT_INLINE(bool) git_parse_ctx_contains(
+	git_parse_ctx *ctx, const char *str, size_t len)
+{
+	return (ctx->line_len >= len && memcmp(ctx->line, str, len) == 0);
+}
+
+void git_parse_advance_line(git_parse_ctx *ctx);
+void git_parse_advance_chars(git_parse_ctx *ctx, size_t char_cnt);
+int git_parse_advance_expected(
+	git_parse_ctx *ctx,
+	const char *expected,
+	size_t expected_len);
+
+#define git_parse_advance_expected_str(ctx, str) \
+	git_parse_advance_expected(ctx, str, strlen(str))
+
+int git_parse_advance_ws(git_parse_ctx *ctx);
+int git_parse_advance_nl(git_parse_ctx *ctx);

src/patch_parse.h

@@ -9,24 +9,15 @@
 
 #include "common.h"
 
+#include "parse.h"
 #include "patch.h"
 
 typedef struct {
 	git_refcount rc;
 
-	/* Original content buffer */
-	const char *content;
-	size_t content_len;
-
 	git_patch_options opts;
 
-	/* The remaining (unparsed) buffer */
-	const char *remain;
-	size_t remain_len;
-
-	const char *line;
-	size_t line_len;
-	size_t line_num;
+	git_parse_ctx parse_ctx;
 } git_patch_parse_ctx;
 
 extern git_patch_parse_ctx *git_patch_parse_ctx_init(