mruby-bin-mirb: add OSC 11 terminal background color detection

Automatically detect terminal background color using OSC 11 escape
sequence to select appropriate syntax highlighting theme (dark/light).

Detection priority: MIRB_THEME env > OSC 11 > COLORFGBG env > dark default.

Co-authored-by: Claude <noreply@anthropic.com>

mrbgems/mruby-bin-mirb/tools/mirb/mirb.c

@@ -37,6 +37,7 @@
 
 #include "mirb_editor.h"
 #include "mirb_completion.h"
+#include "mirb_highlight.h"
 
 /* obsolete configuration */
 #ifdef DISABLE_MIRB_UNDERSCORE
@@ -520,6 +521,11 @@ main(int argc, char **argv)
   mrb_define_global_const(mrb, "ARGV", ARGV);
   mrb_gv_set(mrb, mrb_intern_lit(mrb, "$DEBUG"), mrb_bool_value(args.debug));
 
+  /* Query terminal background color before any output */
+  if (isatty(fileno(stdin)) && isatty(fileno(stdout))) {
+    mirb_highlight_query_terminal();
+  }
+
   print_hint();
 
   cxt = mrb_ccontext_new(mrb);

mrbgems/mruby-bin-mirb/tools/mirb/mirb_highlight.c

@@ -5,6 +5,8 @@
 */
 
 #include "mirb_highlight.h"
+#include "mirb_term.h"
+#include <stdio.h>
 #include <string.h>
 #include <stdlib.h>
 #include <ctype.h>
@@ -140,35 +142,66 @@ print_colored(mirb_highlighter *hl, const char *start, size_t len, mirb_token_ty
   if (*color) printf("%s", reset);
 }
 
+/* Cached terminal background color from pre-query */
+static mirb_bg_color cached_bg_color = MIRB_BG_UNKNOWN;
+static mrb_bool bg_color_queried = FALSE;
+
 /*
- * Detect theme from environment
+ * Pre-query terminal background color
+ * Must be called before any output to avoid response appearing on screen
+ */
+void
+mirb_highlight_query_terminal(void)
+{
+  if (!bg_color_queried) {
+    cached_bg_color = mirb_term_query_bg_color(500);  /* 500ms timeout */
+    bg_color_queried = TRUE;
+  }
+}
+
+/*
+ * Detect theme from terminal background color
+ *
+ * Priority:
+ *   1. MIRB_THEME environment variable (explicit override)
+ *   2. Cached OSC 11 result (from mirb_highlight_query_terminal)
+ *   3. COLORFGBG environment variable (rxvt, some xterm)
+ *   4. Default to dark theme
  */
 mirb_theme
 mirb_highlight_detect_theme(void)
 {
   const char *env;
 
-  /* Check explicit MIRB_THEME first */
+  /* 1. Check explicit MIRB_THEME first (user override) */
   env = getenv("MIRB_THEME");
   if (env) {
     if (strcmp(env, "light") == 0) return MIRB_THEME_LIGHT;
     if (strcmp(env, "dark") == 0) return MIRB_THEME_DARK;
   }
 
-  /* Check COLORFGBG (format: "fg;bg" where bg > 6 usually means light) */
+  /* 2. Use cached OSC 11 result (must call mirb_highlight_query_terminal first) */
+  if (cached_bg_color == MIRB_BG_LIGHT) return MIRB_THEME_LIGHT;
+  if (cached_bg_color == MIRB_BG_DARK) return MIRB_THEME_DARK;
+
+  /* 3. Check COLORFGBG (format: "fg;bg" where bg > 6 usually means light) */
   env = getenv("COLORFGBG");
   if (env) {
     const char *semi = strchr(env, ';');
     if (semi) {
-      int bg = atoi(semi + 1);
+      int bg_color = atoi(semi + 1);
       /* Background colors 7, 15, or high values typically mean light theme */
-      if (bg == 7 || bg == 15 || (bg >= 230 && bg <= 255)) {
+      if (bg_color == 7 || bg_color == 15 || (bg_color >= 230 && bg_color <= 255)) {
         return MIRB_THEME_LIGHT;
       }
+      /* Low values (0-6, 8) typically mean dark theme */
+      if (bg_color <= 8) {
+        return MIRB_THEME_DARK;
+      }
     }
   }
 
-  /* Default to dark theme (more common in terminals) */
+  /* 4. Default to dark theme (more common in terminals) */
   return MIRB_THEME_DARK;
 }
 

mrbgems/mruby-bin-mirb/tools/mirb/mirb_highlight.h

@@ -59,11 +59,17 @@ void mirb_highlight_init(mirb_highlighter *hl, mrb_bool enabled);
 void mirb_highlight_set_theme(mirb_highlighter *hl, mirb_theme theme);
 
 /*
- * Detect theme from environment variables
+ * Detect theme from environment variables and terminal query
  * Returns MIRB_THEME_DARK if cannot detect
  */
 mirb_theme mirb_highlight_detect_theme(void);
 
+/*
+ * Pre-query terminal background color (call before any output)
+ * This caches the result for later use by mirb_highlight_detect_theme()
+ */
+void mirb_highlight_query_terminal(void);
+
 /*
  * Print a line with syntax highlighting
  * Handles multi-line strings/comments by tracking state

mrbgems/mruby-bin-mirb/tools/mirb/mirb_term.c

@@ -348,3 +348,144 @@ mirb_term_clear_below(void)
 {
   printf("\033[J");
 }
+
+#if !defined(_WIN32) && !defined(_WIN64)
+/*
+ * Query terminal background color using OSC 11 escape sequence
+ *
+ * Protocol:
+ *   Send: ESC ] 11 ; ? ESC \   (or BEL instead of ESC \)
+ *   Recv: ESC ] 11 ; rgb:RRRR/GGGG/BBBB ESC \
+ *
+ * The response uses 16-bit color values (0000-FFFF per component).
+ * Some terminals use 8-bit (00-FF) format instead.
+ */
+mirb_bg_color
+mirb_term_query_bg_color(int timeout_ms)
+{
+  struct termios old_term, new_term;
+  char buf[64];
+  size_t total = 0;
+  ssize_t n;
+  int r, g, b;
+  mirb_bg_color result = MIRB_BG_UNKNOWN;
+  fd_set fds;
+  struct timeval tv;
+  const char *p;
+
+  /* Must be a terminal */
+  if (!isatty(STDIN_FILENO) || !isatty(STDOUT_FILENO)) {
+    return MIRB_BG_UNKNOWN;
+  }
+
+  /* Save original terminal settings */
+  if (tcgetattr(STDIN_FILENO, &old_term) < 0) {
+    return MIRB_BG_UNKNOWN;
+  }
+
+  /* Flush any pending input first */
+  tcflush(STDIN_FILENO, TCIFLUSH);
+
+  /* Disable echo and canonical mode for raw read */
+  new_term = old_term;
+  new_term.c_lflag &= ~(ICANON | ECHO | ECHOE | ECHOK | ECHONL);
+  new_term.c_iflag &= ~(IXON | IXOFF | ICRNL);
+  new_term.c_cc[VMIN] = 0;
+  new_term.c_cc[VTIME] = 0;
+
+  /* TCSAFLUSH: flush I/O and apply settings */
+  if (tcsetattr(STDIN_FILENO, TCSAFLUSH, &new_term) < 0) {
+    return MIRB_BG_UNKNOWN;
+  }
+
+  /* Wait for terminal settings to take effect */
+  tcdrain(STDOUT_FILENO);
+
+  /* Send OSC 11 query: ESC ] 11 ; ? ESC \ */
+  if (write(STDOUT_FILENO, "\033]11;?\033\\", 8) != 8) {
+    goto restore;
+  }
+  /* Ensure query is sent to terminal */
+  tcdrain(STDOUT_FILENO);
+  /* Give terminal time to process and respond */
+  usleep(50000);  /* 50ms */
+
+  /* Read response with timeout - loop to get complete response */
+  while (total < sizeof(buf) - 1) {
+    int sel;
+    FD_ZERO(&fds);
+    FD_SET(STDIN_FILENO, &fds);
+    tv.tv_sec = timeout_ms / 1000;
+    tv.tv_usec = (timeout_ms % 1000) * 1000;
+
+    sel = select(STDIN_FILENO + 1, &fds, NULL, NULL, &tv);
+    if (sel < 0) {
+      if (errno == EINTR) continue;  /* Retry on signal interrupt */
+      break;  /* Other error */
+    }
+    if (sel == 0) break;  /* Timeout */
+
+    n = read(STDIN_FILENO, buf + total, sizeof(buf) - 1 - total);
+    if (n <= 0) {
+      if (n < 0 && errno == EINTR) continue;  /* Retry on signal interrupt */
+      break;
+    }
+    total += (size_t)n;
+
+    /* Check for terminator: ESC \ (0x1b 0x5c) or BEL (0x07) */
+    if (total >= 2 && buf[total-2] == '\033' && buf[total-1] == '\\') break;
+    if (total >= 1 && buf[total-1] == '\007') break;
+
+    /* Reduce timeout for subsequent reads */
+    timeout_ms = 10;
+  }
+
+  if (total == 0) {
+    goto restore;
+  }
+  buf[total] = '\0';
+
+  /* Parse response: look for "rgb:" followed by hex values */
+  p = strstr(buf, "rgb:");
+  if (p) {
+    p += 4;
+    /* Try 16-bit format: rgb:RRRR/GGGG/BBBB */
+    if (sscanf(p, "%4x/%4x/%4x", &r, &g, &b) == 3) {
+      /* Normalize to 8-bit range */
+      r >>= 8; g >>= 8; b >>= 8;
+    }
+    /* Try 8-bit format: rgb:RR/GG/BB */
+    else if (sscanf(p, "%2x/%2x/%2x", &r, &g, &b) == 3) {
+      /* Already 8-bit */
+    }
+    else {
+      goto restore;
+    }
+
+    /* Calculate relative luminance (ITU-R BT.709 simplified) */
+    /* Y = 0.2126*R + 0.7152*G + 0.0722*B */
+    /* Scale: 0-255 input, threshold at ~127.5 */
+    int luminance = (2126 * r + 7152 * g + 722 * b) / 10000;
+    result = (luminance < 128) ? MIRB_BG_DARK : MIRB_BG_LIGHT;
+  }
+
+restore:
+  /* Flush any remaining input before restoring terminal */
+  tcflush(STDIN_FILENO, TCIFLUSH);
+  tcsetattr(STDIN_FILENO, TCSAFLUSH, &old_term);
+  return result;
+}
+
+#else /* Windows */
+
+mirb_bg_color
+mirb_term_query_bg_color(int timeout_ms)
+{
+  (void)timeout_ms;
+  /* Windows Terminal supports OSC 11, but implementation requires
+   * different I/O handling. For now, return unknown and rely on
+   * fallback detection methods. */
+  return MIRB_BG_UNKNOWN;
+}
+
+#endif /* _WIN32 */

mrbgems/mruby-bin-mirb/tools/mirb/mirb_term.h

@@ -118,4 +118,20 @@ void mirb_term_get_size(mirb_term *term);
  */
 void mirb_term_flush(void);
 
+/*
+ * Background color detection result
+ */
+typedef enum mirb_bg_color {
+  MIRB_BG_UNKNOWN,   /* could not detect */
+  MIRB_BG_DARK,      /* dark background (luminance < 0.5) */
+  MIRB_BG_LIGHT      /* light background (luminance >= 0.5) */
+} mirb_bg_color;
+
+/*
+ * Query terminal background color using OSC 11
+ * Returns MIRB_BG_UNKNOWN if terminal doesn't respond within timeout
+ * timeout_ms: timeout in milliseconds (recommended: 100)
+ */
+mirb_bg_color mirb_term_query_bg_color(int timeout_ms);
+
 #endif /* MIRB_TERM_H */