syntax and diff background highlighting

Makefile

@@ -366,6 +366,8 @@ TIG_OBJS = \
 	src/grep.o \
 	src/ui.o \
 	src/apps.o \
+	src/ansi.o \
+	src/color_mode.o \
 	$(GRAPH_OBJS) \
 	$(COMPAT_OBJS)
 
@@ -374,10 +376,16 @@ src/tig: $(TIG_OBJS)
 TEST_GRAPH_OBJS = test/tools/test-graph.o src/string.o src/util.o src/io.o $(GRAPH_OBJS) $(COMPAT_OBJS)
 test/tools/test-graph: $(TEST_GRAPH_OBJS)
 
+TEST_ANSI_OBJS = test/tools/test-ansi.o src/ansi.o $(COMPAT_OBJS)
+test/tools/test-ansi: $(TEST_ANSI_OBJS)
+
+TEST_COLOR_MODE_OBJS = test/tools/test-color-mode.o src/color_mode.o $(COMPAT_OBJS)
+test/tools/test-color-mode: $(TEST_COLOR_MODE_OBJS)
+
 DOC_GEN_OBJS = tools/doc-gen.o src/string.o src/types.o src/util.o src/request.o $(COMPAT_OBJS)
 tools/doc-gen: $(DOC_GEN_OBJS)
 
-OBJS = $(sort $(TIG_OBJS) $(TEST_GRAPH_OBJS) $(DOC_GEN_OBJS))
+OBJS = $(sort $(TIG_OBJS) $(TEST_GRAPH_OBJS) $(TEST_ANSI_OBJS) $(TEST_COLOR_MODE_OBJS) $(DOC_GEN_OBJS))
 
 DEPS_CFLAGS ?= -MMD -MP -MF .deps/$*.d
 

README.adoc

@@ -46,3 +46,79 @@ link:INSTALL.{docext}[the installation instructions].
 
 News about releases and latest features and bug fixes are found in
 link:NEWS.{docext}[the release notes].
+
+Fork-specific features
+----------------------
+
+This fork (`me/syntax-and-diff-bg-highlighting`) adds two diff-view changes
+on top of upstream tig:
+
+* Full-row red/green background bars on `-`/`+` lines (in place of upstream's
+  single-glyph coloring).
+* Syntax-highlighted diff text, piped through https://github.com/sharkdp/bat[`bat`]
+  for the file's language. Requires `bat` on `$PATH`.
+
+Recommended terminal setup
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The background bars look best in 24-bit direct color. The bar still works
+at lower color tiers but uses coarser hues — see "Graceful degradation"
+below.
+
+In a normal terminal (alacritty, kitty, foot, xterm, wezterm, etc.):
+
+[source,sh]
+----
+export TERM=xterm-direct
+----
+
+Inside neovim's `:terminal` (libvterm-based):
+
+[source,sh]
+----
+export TERM=xterm-direct2
+----
+
+Why the split: stock `xterm-direct`'s `setab`/`setaf` strings emit SGR
+sequences in the ITU T.416 pedantic form `CSI 48:2::R:G:B m` (two colons
+after `2`). libvterm doesn't parse the empty colorspace-ID slot and
+mis-reads the RGB triplet, producing garbled backgrounds. The
+`xterm-direct2` entry drops that slot — `CSI 48:2:R:G:B m` — which every
+libvterm-based terminal handles correctly. xterm and other emulators
+parse either form.
+
+If `xterm-direct`/`xterm-direct2` is not present on your system, the
+ncurses package usually provides it under `/usr/share/terminfo` or
+`/opt/homebrew/opt/ncurses/share/terminfo` (`infocmp xterm-direct` to
+verify). On Linux, the brewed ncurses ships these:
+
+[source,sh]
+----
+export TERMINFO_DIRS=/home/linuxbrew/.linuxbrew/share/terminfo:
+----
+
+Graceful degradation
+~~~~~~~~~~~~~~~~~~~~
+
+The diff-bg code picks colors at runtime based on ncurses' `COLORS`
+report. There are three tiers:
+
+[cols="1,2,3"]
+|===
+| Tier | Condition | Behaviour
+
+| Truecolor   | `COLORS >= 16777216`
+| Renders the bar in the GitHub-style dark red / dark green RGB values
+  (`#5f0000` / `#005f00`). diff-highlight emphasis ranges get brighter
+  variants (`#870000` / `#5f875f`).
+| 256-palette | `COLORS >= 256`
+| Renders the bar using the xterm-256 palette indices the truecolor RGB
+  values are derived from (22, 52, 65, 88). Hues match exactly when the
+  terminal uses the canonical xterm color cube.
+| Basic       | `COLORS < 256`
+| Falls back to the 8 basic ANSI colors. The dark/medium emphasis
+  distinction is lost (52 and 88 both render as red); the bar is still
+  full-row.
+|===
+
+The classification logic is exercised by `test/tools/test-color-mode.c`.

doc/tigrc.5.adoc

@@ -291,6 +291,23 @@ The following variables can be set:
 	regions are governed by `color diff-add-highlight` and
 	`color diff-del-highlight`.
 
+'syntax-highlight' (mixed)::
+
+	Whether to syntax-highlight file contents in diff, stage, blob, and
+	pager views using an external highlighter. Defaults to false. When set
+	to true then 'bat' is used, else the option value is used as the path
+	to the highlighter command. Requires a terminal with 256-color support
+	(TERM=xterm-256color or equivalent); silently disabled on terminals
+	with fewer than 256 colors. +
+	 +
+	When enabled, diff views show syntax-colored code on dark green
+	(additions) or dark red (deletions) backgrounds. Can be combined with
+	'diff-highlight' for intra-line change emphasis. Long lines are wrapped
+	with a 2-space indent on continuation lines. +
+	 +
+	The highlighter must accept `--color=always --style=plain --paging=never
+	--file-name=<name> -` arguments and read from stdin.
+
 'diff-indicator' (bool)::
 
 	Show +/- signs in the diff view. On by default.

include/tig/ansi.h

@@ -0,0 +1,69 @@
+/* Copyright (c) 2006-2026 Jonas Fonseca <jonas.fonseca@gmail.com>
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of
+ * the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ */
+
+#ifndef TIG_ANSI_H
+#define TIG_ANSI_H
+
+#include "tig/tig.h"
+
+#define ANSI_MAX_SPANS	512
+
+/* Parameters in one SGR sequence.  The colon form needs six per color, so
+ * a reset plus attributes plus fg and bg can reach the high teens. */
+#define ANSI_MAX_SGR_CODES	32
+
+enum ansi_color_type {
+	ANSI_COLOR_DEFAULT,
+	ANSI_COLOR_BASIC,	/* 0-7: standard ANSI colors */
+	ANSI_COLOR_256,		/* 0-255: extended palette */
+	ANSI_COLOR_RGB		/* 24-bit truecolor */
+};
+
+struct ansi_color {
+	enum ansi_color_type type;
+	union {
+		int index;
+		struct { unsigned char r, g, b; } rgb;
+	};
+};
+
+struct ansi_span {
+	struct ansi_color fg;
+	struct ansi_color bg;
+	int attr;		/* ncurses attributes: A_BOLD, A_UNDERLINE, etc. */
+	size_t offset;		/* byte offset in stripped text */
+	size_t length;		/* byte length of this span */
+};
+
+/*
+ * Parse a line containing ANSI escape sequences.
+ *
+ * Strips escape codes from `raw` and writes plain text to `stripped`.
+ * Records color/attribute spans in `spans`.
+ *
+ * Returns the number of spans written, or -1 on error.
+ */
+int ansi_parse_line(const char *raw, char *stripped, size_t stripped_size,
+		    struct ansi_span *spans, int max_spans);
+
+/*
+ * Returns true if the string contains ANSI escape sequences.
+ */
+static inline bool
+ansi_has_escapes(const char *text)
+{
+	return text && strchr(text, 0x1b) != NULL;
+}
+
+#endif
+/* vim: set ts=8 sw=8 noexpandtab: */

include/tig/apps.h

@@ -33,6 +33,12 @@ struct app_external {
 
 struct app_external *app_diff_highlight_load(const char *query);
 
+/*
+ * syntax-highlight (bat)
+ */
+
+struct app_external *app_syntax_highlight_load(const char *query, const char *filename);
+
 #endif
 
 /* vim: set ts=8 sw=8 noexpandtab: */

include/tig/color_mode.h

@@ -0,0 +1,57 @@
+/* Copyright (c) 2006-2026 Jonas Fonseca <jonas.fonseca@gmail.com>
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of
+ * the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ */
+
+#ifndef TIG_COLOR_MODE_H
+#define TIG_COLOR_MODE_H
+
+#include "tig/ansi.h"
+
+/*
+ * Runtime color-support tier.
+ *
+ * In direct-color mode (TERM=*-direct) ncurses treats color *numbers*
+ * as packed 24-bit RGB. Passing a 256-palette index like 52 yields
+ * RGB(0,0,52) — nearly black — instead of dark red. Code that wants
+ * a specific hue must translate palette indices to real RGB.
+ */
+enum tig_color_mode {
+	TIG_COLOR_BASIC,	/* < 256: coarse 8-color ANSI fallback */
+	TIG_COLOR_256,		/* 256-palette indices work as-is */
+	TIG_COLOR_TRUECOLOR	/* numbers are packed 24-bit RGB */
+};
+
+/*
+ * Pure classification of an ncurses COLORS count into a tier.
+ * Separated from the ncurses dependency so it is unit-testable.
+ */
+enum tig_color_mode tig_classify_color_mode(int ncurses_colors);
+
+/*
+ * Translate an ANSI color descriptor to the int value that should be
+ * handed to init_extended_pair under the given color mode.
+ *
+ *  - Basic ANSI 0-7 are returned unchanged in every mode (terminfo
+ *    setaf/setab entries special-case them even in direct mode).
+ *  - 256-palette indices are returned unchanged in TIG_COLOR_256,
+ *    converted to packed RGB in TIG_COLOR_TRUECOLOR, and approximated
+ *    to a basic ANSI color in TIG_COLOR_BASIC.
+ *  - Truecolor (24-bit) input is packed into 0xRRGGBB in TRUECOLOR
+ *    mode, mapped to the nearest xterm-256 index in PALETTE_256, and
+ *    coarsened to basic ANSI in BASIC mode.
+ *  - ANSI_COLOR_DEFAULT always returns -1 (ncurses default-color).
+ */
+int ansi_color_to_ncurses_for_mode(const struct ansi_color *color,
+				   enum tig_color_mode mode);
+
+#endif
+/* vim: set ts=8 sw=8 noexpandtab: */

include/tig/diff.h

@@ -16,6 +16,47 @@
 
 #include "tig/view.h"
 
+/*
+ * Deferred syntax highlighting.
+ *
+ * Running the highlighter while the diff is read costs a fork+exec per file,
+ * which dominates the load time of a diff touching many files.  Instead each
+ * content line is recorded and rendered plain (keeping the diff background so
+ * only foregrounds change later), and the highlighter runs when a line is
+ * about to be drawn.  Files nobody scrolls to never spawn a process.
+ *
+ * Each file keeps its own highlighter and is fed strictly in order, so the
+ * lexer state always matches the real file.  `fed` counts lines written to the
+ * current process and resets when it is recycled; `applied` counts lines whose
+ * colors have been installed and only ever grows.
+ */
+struct diff_lazy_line {
+	char *data;			/* original diff line, prefix included */
+	unsigned long lineno;		/* first view line for this diff line */
+	unsigned int segs;		/* view lines it wrapped into */
+	enum line_type type;
+	bool indicator_stripped;	/* caller already removed the +/- */
+};
+
+struct diff_lazy_file {
+	char path[SIZEOF_STR];
+	pid_t pid;
+	int write_fd;
+	FILE *read_fp;
+	bool failed;			/* highlighter gone; leave rest plain */
+	unsigned int used;		/* LRU stamp for pipe recycling */
+	unsigned int fed;		/* lines written to the current pipe */
+	unsigned int applied;		/* lines whose colors are installed */
+	struct diff_lazy_line *lines;
+	size_t nlines;
+};
+
+/* Reverse map from a view line back to the diff line that produced it */
+struct diff_lazy_map {
+	int file;			/* index into syntax_files, or -1 */
+	unsigned int index;		/* line index within that file */
+};
+
 struct diff_state {
 	bool after_commit_title;
 	bool after_diff;
@@ -30,6 +71,15 @@ struct diff_state {
 	unsigned int lineno;
 	struct position pos;
 	struct io view_io;
+	/* Syntax highlighting state */
+	bool syntax_highlight;
+	char syntax_file[SIZEOF_STR];	/* Current file from diff +++ header */
+	struct diff_lazy_file *syntax_files;
+	size_t syntax_nfiles;
+	int syntax_current;		/* file being read, or -1 */
+	unsigned int syntax_clock;	/* ticks the LRU stamps */
+	struct diff_lazy_map *syntax_map;
+	size_t syntax_map_size;		/* view lines covered by syntax_map */
 };
 
 enum request diff_common_edit(struct view *view, enum request request, struct line *line);
@@ -41,6 +91,23 @@ void diff_save_line(struct view *view, struct diff_state *state, enum open_flags
 void diff_restore_line(struct view *view, struct diff_state *state);
 enum status_code diff_init_highlight(struct view *view, struct diff_state *state);
 bool diff_done_highlight(struct diff_state *state);
+void diff_init_syntax_highlight(struct diff_state *state);
+void diff_done_syntax_highlight(struct diff_state *state);
+
+/*
+ * Draw and teardown hooks shared by the diff-like views, which defer syntax
+ * highlighting until a line is about to be drawn.
+ */
+bool diff_draw(struct view *view, struct line *line, unsigned int lineno);
+void diff_done(struct view *view);
+
+/*
+ * A wrapped diff line occupies several view lines.  Code that turns view lines
+ * back into a patch must skip the continuations and use the original text.
+ */
+bool diff_is_wrapped_continuation(struct view *view, struct line *line);
+const char *diff_original_text(struct view *view, struct line *line,
+			       char *buf, size_t bufsize);
 
 unsigned int diff_get_lineno(struct view *view, struct line *line, bool old);
 const char *diff_get_pathname(struct view *view, struct line *line, bool old);

include/tig/line.h

@@ -154,5 +154,12 @@ get_line_attr(const char *prefix, enum line_type type)
 	return COLOR_PAIR(COLOR_ID(info->color_pair)) | info->attr;
 }
 
+/*
+ * Dynamic color pair allocation for syntax highlighting.
+ */
+struct ansi_color;
+int ansi_color_to_ncurses(const struct ansi_color *color);
+int get_dynamic_color_pair(const struct ansi_color *fg, const struct ansi_color *bg);
+
 #endif
 /* vim: set ts=8 sw=8 noexpandtab: */

include/tig/options.h

@@ -84,6 +84,7 @@ typedef struct view_column *view_settings;
 	_(status_show_untracked_dirs,	bool,			VIEW_STATUS_LIKE) \
 	_(status_show_untracked_files,	bool,			VIEW_STATUS_LIKE) \
 	_(status_view,			view_settings,		VIEW_NO_FLAGS) \
+	_(syntax_highlight,		const char *,		VIEW_NO_FLAGS) \
 	_(tab_size,			int,			VIEW_NO_FLAGS) \
 	_(tree_view,			view_settings,		VIEW_NO_FLAGS) \
 	_(truncation_delimiter,		const char *,		VIEW_NO_FLAGS) \

include/tig/pager.h

@@ -18,6 +18,7 @@
 
 bool pager_get_column_data(struct view *view, const struct line *line, struct view_column_data *column_data);
 bool pager_common_read(struct view *view, const char *data, enum line_type type, struct line **line);
+bool pager_add_ansi_line(struct view *view, const char *data, enum line_type type);
 enum request pager_request(struct view *view, enum request request, struct line *line);
 void pager_select(struct view *view, struct line *line);
 

include/tig/view.h

@@ -28,6 +28,9 @@ struct view_ops;
 struct box_cell {
 	enum line_type type;
 	size_t length;
+	unsigned int direct : 1;	/* If set, use color_pair/attr directly */
+	int color_pair;			/* Dynamic ncurses color pair ID */
+	int attr;			/* A_BOLD | A_UNDERLINE | ... */
 };
 
 struct box {