parse: implement `git_parse_peek`

Some code parts need to inspect the next few bytes without actually
consuming it yet, for example to examine what content it has to expect
next. Create a new function `git_parse_peek` which returns the next byte
without modifying the parsing context and use it at multiple call sites.

src/parse.c

@@ -94,3 +94,25 @@ int git_parse_advance_digit(int64_t *out, git_parse_ctx *ctx, int base)
 	git_parse_advance_chars(ctx, (end - ctx->line));
 	return 0;
 }
+
+int git_parse_peek(char *out, git_parse_ctx *ctx, int flags)
+{
+	size_t remain = ctx->line_len;
+	const char *ptr = ctx->line;
+
+	while (remain) {
+		char c = *ptr;
+
+		if ((flags & GIT_PARSE_PEEK_SKIP_WHITESPACE) &&
+		    git__isspace(c)) {
+			remain--;
+			ptr++;
+			continue;
+		}
+
+		*out = c;
+		return 0;
+	}
+
+	return -1;
+}

src/parse.h

@@ -48,3 +48,9 @@ int git_parse_advance_expected(
 int git_parse_advance_ws(git_parse_ctx *ctx);
 int git_parse_advance_nl(git_parse_ctx *ctx);
 int git_parse_advance_digit(int64_t *out, git_parse_ctx *ctx, int base);
+
+enum GIT_PARSE_PEEK_FLAGS {
+	GIT_PARSE_PEEK_SKIP_WHITESPACE = (1 << 0)
+};
+
+int git_parse_peek(char *out, git_parse_ctx *ctx, int flags);

src/patch_parse.c

@@ -142,6 +142,8 @@ static int parse_header_oid(
 static int parse_header_git_index(
 	git_patch_parsed *patch, git_patch_parse_ctx *ctx)
 {
+	char c;
+
 	if (parse_header_oid(&patch->base.delta->old_file.id,
 			&patch->base.delta->old_file.id_abbrev, ctx) < 0 ||
 		git_parse_advance_expected_str(&ctx->parse_ctx, "..") < 0 ||
@@ -149,7 +151,7 @@ static int parse_header_git_index(
 			&patch->base.delta->new_file.id_abbrev, ctx) < 0)
 		return -1;
 
-	if (ctx->parse_ctx.line_len > 0 && ctx->parse_ctx.line[0] == ' ') {
+	if (git_parse_peek(&c, &ctx->parse_ctx, 0) == 0 && c == ' ') {
 		uint16_t mode;
 
 		git_parse_advance_chars(&ctx->parse_ctx, 1);
@@ -458,6 +460,7 @@ static int parse_hunk_header(
 	git_patch_parse_ctx *ctx)
 {
 	const char *header_start = ctx->parse_ctx.line;
+	char c;
 
 	hunk->hunk.old_lines = 1;
 	hunk->hunk.new_lines = 1;
@@ -466,7 +469,7 @@ static int parse_hunk_header(
 		parse_int(&hunk->hunk.old_start, ctx) < 0)
 		goto fail;
 
-	if (ctx->parse_ctx.line_len > 0 && ctx->parse_ctx.line[0] == ',') {
+	if (git_parse_peek(&c, &ctx->parse_ctx, 0) == 0 && c == ',') {
 		if (git_parse_advance_expected_str(&ctx->parse_ctx, ",") < 0 ||
 			parse_int(&hunk->hunk.old_lines, ctx) < 0)
 			goto fail;
@@ -476,7 +479,7 @@ static int parse_hunk_header(
 		parse_int(&hunk->hunk.new_start, ctx) < 0)
 		goto fail;
 
-	if (ctx->parse_ctx.line_len > 0 && ctx->parse_ctx.line[0] == ',') {
+	if (git_parse_peek(&c, &ctx->parse_ctx, 0) == 0 && c == ',') {
 		if (git_parse_advance_expected_str(&ctx->parse_ctx, ",") < 0 ||
 			parse_int(&hunk->hunk.new_lines, ctx) < 0)
 			goto fail;
@@ -523,6 +526,7 @@ static int parse_hunk_body(
 		!git_parse_ctx_contains_s(&ctx->parse_ctx, "@@ -");
 		git_parse_advance_line(&ctx->parse_ctx)) {
 
+		char c;
 		int origin;
 		int prefix = 1;
 
@@ -532,7 +536,9 @@ static int parse_hunk_body(
 			goto done;
 		}
 
-		switch (ctx->parse_ctx.line[0]) {
+		git_parse_peek(&c, &ctx->parse_ctx, 0);
+
+		switch (c) {
 		case '\n':
 			prefix = 0;
 
@@ -683,9 +689,11 @@ static int parse_patch_binary_side(
 	}
 
 	while (ctx->parse_ctx.line_len) {
-		char c = ctx->parse_ctx.line[0];
+		char c;
 		size_t encoded_len, decoded_len = 0, decoded_orig = decoded.size;
 
+		git_parse_peek(&c, &ctx->parse_ctx, 0);
+
 		if (c == '\n')
 			break;
 		else if (c >= 'A' && c <= 'Z')