Font fallback: Fix the font used to render a character sometimes dependent on the order in which characters appear on screen

We ameliorate the performance hit by storing a hash table mapping cell
text to the loaded fallback font index so that lookups for previously
seen text are still fast.

Fixes #6865

docs/changelog.rst

@@ -61,6 +61,8 @@ Detailed list of changes
 
 - macOS: Fix returning from full screen via the button when the titlebar is hidden not hiding the buttons (:iss:`6883`)
 
+- Font fallback: Fix the font used to render a character sometimes dependent on the order in which characters appear on screen (:iss:`6865`)
+
 
 0.31.0 [2023-11-08]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

kitty/core_text.m

@@ -240,7 +240,7 @@ find_substitute_face(CFStringRef str, CTFontRef old_font, CPUCell *cpu_cell) {
                 new_font = manually_search_fallback_fonts(old_font, cpu_cell);
                 if (new_font) return new_font;
             }
-            PyErr_SetString(PyExc_ValueError, "Failed to find fallback CTFont other than the LastResort font");
+            PyErr_Format(PyExc_ValueError, "Failed to find fallback CTFont other than the LastResort font for: %s", [(NSString *)str UTF8String]);
             return NULL;
         }
         return new_font;
@@ -270,7 +270,22 @@ create_fallback_face(PyObject *base_face, CPUCell* cell, bool UNUSED bold, bool
     }
     else { search_for_fallback(); }
     if (new_font == NULL) return NULL;
-    return (PyObject*)ct_face(new_font, fg);
+    NSURL *url = (NSURL*)CTFontCopyAttribute(new_font, kCTFontURLAttribute);
+    const char *font_path = [[url path] UTF8String];
+    ssize_t idx = -1;
+    PyObject *q, *ans = NULL;
+    while ((q = iter_fallback_faces(fg, &idx))) {
+        CTFace *qf = (CTFace*)q;
+        const char *qpath;
+        if (qf->path && (qpath = PyUnicode_AsUTF8(qf->path)) && strcmp(qpath, font_path) == 0) {
+            ans = PyLong_FromSsize_t(idx);
+            break;
+        }
+    }
+    [url release];
+    if (ans == NULL) return (PyObject*)ct_face(new_font, fg);
+    CFRelease(new_font);
+    return ans;
 }
 
 unsigned int

kitty/fontconfig.c

@@ -450,7 +450,15 @@ create_fallback_face(PyObject UNUSED *base_face, CPUCell* cell, bool bold, bool
     size_t num = cell_as_unicode_for_fallback(cell, char_buf);
     add_charset(pat, num);
     PyObject *d = _fc_match(pat);
-    if (d) { ans = face_from_descriptor(d, fg); Py_CLEAR(d); }
+    if (d) {
+        ssize_t idx = -1;
+        PyObject *q;
+        while ((q = iter_fallback_faces(fg, &idx))) {
+            if (face_equals_descriptor(q, d)) { ans = PyLong_FromSsize_t(idx); Py_CLEAR(d); goto end; }
+        }
+        ans = face_from_descriptor(d, fg);
+        Py_CLEAR(d);
+    }
 end:
     if (pat != NULL) FcPatternDestroy(pat);
     return ans;

kitty/fonts.c

@@ -12,6 +12,7 @@
 #include "unicode-data.h"
 #include "charsets.h"
 #include "glyph-cache.h"
+#include "kitty-uthash.h"
 
 #define MISSING_GLYPH (NUM_UNDERLINE_STYLES + 2)
 #define MAX_NUM_EXTRA_GLYPHS_PUA 4u
@@ -71,6 +72,12 @@ typedef struct Canvas {
     unsigned current_cells, alloced_cells;
 } Canvas;
 
+typedef struct fallback_font_map {
+    const char *cell_text;
+    size_t font_idx;
+    UT_hash_handle hh;
+} fallback_font_map_t;
+
 typedef struct {
     FONTS_DATA_HEAD
     id_type id;
@@ -80,6 +87,7 @@ typedef struct {
     Font *fonts;
     Canvas canvas;
     GPUSpriteTracker sprite_tracker;
+    fallback_font_map_t *fallback_font_map;
 } FontGroup;
 
 static FontGroup* font_groups = NULL;
@@ -152,6 +160,15 @@ static void
 del_font_group(FontGroup *fg) {
     free(fg->canvas.buf); fg->canvas.buf = NULL; fg->canvas = (Canvas){0};
     fg->sprite_map = free_sprite_map(fg->sprite_map);
+    if (fg->fallback_font_map) {
+        fallback_font_map_t *current, *tmp;
+        HASH_ITER(hh, fg->fallback_font_map, current, tmp) {
+            free((void*)current->cell_text);
+            HASH_DEL(fg->fallback_font_map, current);
+            free(current);
+        }
+        fg->fallback_font_map = NULL;
+    }
     for (size_t i = 0; i < fg->fonts_count; i++) del_font(fg->fonts + i);
     free(fg->fonts); fg->fonts = NULL;
 }
@@ -451,6 +468,16 @@ output_cell_fallback_data(CPUCell *cell, bool bold, bool italic, bool emoji_pres
     printf("\n");
 }
 
+PyObject*
+iter_fallback_faces(FONTS_DATA_HANDLE fgh, ssize_t *idx) {
+    FontGroup *fg = (FontGroup*)fgh;
+    if (*idx + 1 < (ssize_t)fg->fallback_fonts_count) {
+        *idx += 1;
+        return fg->fonts[fg->first_fallback_font_idx + *idx].face;
+    }
+    return NULL;
+}
+
 static ssize_t
 load_fallback_font(FontGroup *fg, CPUCell *cell, bool bold, bool italic, bool emoji_presentation) {
     if (fg->fallback_fonts_count > 100) { log_error("Too many fallback fonts"); return MISSING_FONT; }
@@ -464,6 +491,7 @@ load_fallback_font(FontGroup *fg, CPUCell *cell, bool bold, bool italic, bool em
     if (face == NULL) { PyErr_Print(); return MISSING_FONT; }
     if (face == Py_None) { Py_DECREF(face); return MISSING_FONT; }
     if (global_state.debug_font_fallback) output_cell_fallback_data(cell, bold, italic, emoji_presentation, face, true);
+    if (PyLong_Check(face)) { ssize_t ans = fg->first_fallback_font_idx + PyLong_AsSsize_t(face); Py_DECREF(face); return ans; }
     set_size_for_face(face, fg->cell_height, true, (FONTS_DATA_HANDLE)fg);
 
     ensure_space_for(fg, fonts, Font, fg->fonts_count + 1, fonts_capacity, 5, true);
@@ -495,17 +523,24 @@ fallback_font(FontGroup *fg, CPUCell *cpu_cell, GPUCell *gpu_cell) {
     bool bold = gpu_cell->attrs.bold;
     bool italic = gpu_cell->attrs.italic;
     bool emoji_presentation = has_emoji_presentation(cpu_cell, gpu_cell);
-
-    // Check if one of the existing fallback fonts has this text
-    for (size_t i = 0, j = fg->first_fallback_font_idx; i < fg->fallback_fonts_count; i++, j++)  {
-        Font *ff = fg->fonts +j;
-        if (ff->bold == bold && ff->italic == italic && ff->emoji_presentation == emoji_presentation && has_cell_text(ff, cpu_cell)) {
-            if (global_state.debug_font_fallback) output_cell_fallback_data(cpu_cell, bold, italic, emoji_presentation, ff->face, false);
-            return j;
+    char cell_text[6 + arraysz(cpu_cell->cc_idx) * 4];
+    const size_t cell_text_len = cell_as_utf8(cpu_cell, true, cell_text, ' ');
+    if (fg->fallback_font_map) {
+        fallback_font_map_t *s;
+        HASH_FIND_STR(fg->fallback_font_map, cell_text, s);
+        /* printf("cache %s\n", (s ? "hit" : "miss")); */
+        if (s) return s->font_idx;
+    }
+    ssize_t idx = load_fallback_font(fg, cpu_cell, bold, italic, emoji_presentation);
+    fallback_font_map_t *ffm = calloc(1, sizeof(fallback_font_map_t));
+    if (ffm) {
+        ffm->font_idx = idx;
+        ffm->cell_text = strndup(cell_text, cell_text_len);
+        if (ffm->cell_text) {
+            HASH_ADD_KEYPTR(hh, fg->fallback_font_map, ffm->cell_text, cell_text_len, ffm);
         }
     }
-
-    return load_fallback_font(fg, cpu_cell, bold, italic, emoji_presentation);
+    return idx;
 }
 
 static ssize_t

kitty/fonts.h

@@ -30,6 +30,8 @@ PyObject* create_fallback_face(PyObject *base_face, CPUCell* cell, bool bold, bo
 PyObject* specialize_font_descriptor(PyObject *base_descriptor, FONTS_DATA_HANDLE);
 PyObject* face_from_path(const char *path, int index, FONTS_DATA_HANDLE);
 PyObject* face_from_descriptor(PyObject*, FONTS_DATA_HANDLE);
+PyObject* iter_fallback_faces(FONTS_DATA_HANDLE fgh, ssize_t *idx);
+bool face_equals_descriptor(PyObject *face_, PyObject *descriptor);
 const char* postscript_name_for_face(const PyObject*);
 
 void sprite_tracker_current_layout(FONTS_DATA_HANDLE data, unsigned int *x, unsigned int *y, unsigned int *z);

kitty/freetype.c

@@ -218,6 +218,17 @@ set_load_error(const char *path, int error) {
     return NULL;
 }
 
+bool
+face_equals_descriptor(PyObject *face_, PyObject *descriptor) {
+    Face *face = (Face*)face_;
+    PyObject *t = PyDict_GetItemString(descriptor, "path");
+    if (!t) return false;
+    if (PyObject_RichCompareBool(face->path, t, Py_EQ) != 1) return false;
+    t = PyDict_GetItemString(descriptor, "index");
+    if (t && PyLong_AsLong(t) != face->index) return false;
+    return true;
+}
+
 PyObject*
 face_from_descriptor(PyObject *descriptor, FONTS_DATA_HANDLE fg) {
 #define D(key, conv, missing_ok) { \
@@ -285,7 +296,6 @@ repr(Face *self) {
     );
 }
 
-
 const char*
 postscript_name_for_face(const PyObject *face_) {
     const Face *self = (const Face*)face_;
@@ -723,7 +733,6 @@ static PyMethodDef methods[] = {
     {NULL}  /* Sentinel */
 };
 
-
 PyTypeObject Face_Type = {
     PyVarObject_HEAD_INIT(NULL, 0)
     .tp_name = "fast_data_types.Face",