From 8ffc556d441013e06236f144020b91ea4e3a3f86 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Daniel=20Ekl=C3=B6f?= <daniel@ekloef.se>
Date: Tue, 31 Aug 2021 19:42:22 +0200
Subject: [PATCH] main: verify primary font is monospaced at startup

Load a couple of ASCII glyphs and check if their advance widths
matches. If not, warn the user that the font is probably not
monospaced.

This can be disabled by setting tweak.font-monospace-warn=no.

Closes #704.
---
 config.c           | 52 ++++++++++++++++++++++++++++++++++++++++++++++
 config.h           |  4 ++++
 doc/foot.ini.5.scd | 11 ++++++++++
 main.c             |  5 +++++
 4 files changed, 72 insertions(+)

diff --git a/config.c b/config.c
index ac6dd24d..ca1278df 100644
--- a/config.c
+++ b/config.c
@@ -2452,6 +2452,9 @@ parse_section_tweak(
                      conf->tweak.box_drawing_solid_shades ? "yes" : "no");
     }
 
+    else if (strcmp(key, "font-monospace-warn") == 0)
+        conf->tweak.font_monospace_warn = str_to_bool(value);
+
     else {
         LOG_AND_NOTIFY_ERR("%s:%u: [tweak]: %s: invalid key", path, lineno, key);
         return false;
@@ -2957,6 +2960,7 @@ config_load(struct config *conf, const char *conf_path,
             .damage_whole_window = false,
             .box_drawing_base_thickness = 0.04,
             .box_drawing_solid_shades = true,
+            .font_monospace_warn = true,
         },
 
         .notifications = tll_init(),
@@ -3356,3 +3360,51 @@ config_font_list_destroy(struct config_font_list *font_list)
     font_list->count = 0;
     font_list->arr = NULL;
 }
+
+
+bool
+check_if_font_is_monospaced(const char *pattern,
+                            user_notifications_t *notifications)
+{
+    struct fcft_font *f = fcft_from_name(
+        1, (const char *[]){pattern}, ":size=8");
+
+    if (f == NULL)
+        return true;
+
+    static const wchar_t chars[] = {L'a', L'i', L'l', L'M', L'W'};
+
+    bool is_monospaced = true;
+    int last_width = -1;
+
+    for (size_t i = 0; i < sizeof(chars) / sizeof(chars[0]); i++) {
+        const struct fcft_glyph *g = fcft_glyph_rasterize(
+            f, chars[i], FCFT_SUBPIXEL_NONE);
+
+        if (g == NULL)
+            continue;
+
+        if (last_width >= 0 && g->advance.x != last_width) {
+            LOG_WARN("%s: font does not appear to be monospace; "
+                     "check your config, or disable this warning by "
+                     "setting [tweak].font-monospace-warn=no",
+                     pattern);
+
+            user_notification_add(
+                notifications,
+                USER_NOTIFICATION_WARNING,
+                "%s: font does not appear to be monospace; "
+                "check your config, or disable this warning by "
+                "settting \033[1m[tweak].font-monospace-warn=no\033[22m",
+                pattern);
+
+            is_monospaced = false;
+            break;
+        }
+
+        last_width = g->advance.x;
+    }
+
+    fcft_destroy(f);
+    return is_monospaced;
+}
diff --git a/config.h b/config.h
index 40daaf3b..6646cca3 100644
--- a/config.h
+++ b/config.h
@@ -253,6 +253,7 @@ struct config {
         off_t max_shm_pool_size;
         float box_drawing_base_thickness;
         bool box_drawing_solid_shades;
+        bool font_monospace_warn;
     } tweak;
 
     user_notifications_t notifications;
@@ -269,3 +270,6 @@ struct config *config_clone(const struct config *old);
 
 bool config_font_parse(const char *pattern, struct config_font *font);
 void config_font_list_destroy(struct config_font_list *font_list);
+
+bool check_if_font_is_monospaced(
+    const char *pattern, user_notifications_t *notifications);
diff --git a/doc/foot.ini.5.scd b/doc/foot.ini.5.scd
index 65a3a4dc..a124316e 100644
--- a/doc/foot.ini.5.scd
+++ b/doc/foot.ini.5.scd
@@ -1022,6 +1022,17 @@ any of these options.
 	
 	Default: _double-width_
 
+*font-monospace-warn*
+	Boolean. When enabled, foot will use heuristics to try to verify
+	the primary font is a monospace font, and warn if it is not.
+	
+	Disable this if you still want to use the font, even if foot
+	thinks it is not monospaced.
+	
+	You may also want to disable it to get slightly faster startup times.
+	
+	Default: _yes_
+
 *max-shm-pool-size-mb*
 	This option controls the amount of virtual address space used by
 	the pixmap memory to which the terminal screen content is
diff --git a/main.c b/main.c
index b036b9f6..4f7b027d 100644
--- a/main.c
+++ b/main.c
@@ -480,6 +480,11 @@ main(int argc, char *const *argv)
     conf.presentation_timings = presentation_timings;
     conf.hold_at_exit = hold;
 
+    if (conf.tweak.font_monospace_warn && conf.fonts[0].count > 0) {
+        check_if_font_is_monospaced(
+            conf.fonts[0].arr[0].pattern, &conf.notifications);
+    }
+
     struct fdm *fdm = NULL;
     struct reaper *reaper = NULL;
     struct wayland *wayl = NULL;