From 26712247add19a54d4671f2c0e50234905c2c608 Mon Sep 17 00:00:00 2001
From: tkchia <tkchia-cosmo@gmx.com>
Date: Sat, 10 Sep 2022 06:18:43 +0000
Subject: [PATCH] Bare metal VGA: simplify treatment of background color codes

---
 libc/vga/tty.c | 76 +++++++++++++++++++++++---------------------------
 1 file changed, 35 insertions(+), 41 deletions(-)

diff --git a/libc/vga/tty.c b/libc/vga/tty.c
index 84bac011f..3094c9db6 100644
--- a/libc/vga/tty.c
+++ b/libc/vga/tty.c
@@ -261,15 +261,29 @@ static void TtySetCodepage(struct Tty *tty, char id) {
   }
 }
 
+/**
+ * Map the currently active foreground & background colors & terminal
+ * configuration to a VGA text character attribute byte.
+ *
+ * @see VGA_USE_BLINK macro (libc/vga/vga.internal.h)
+ * @see drivers/tty/vt/vt.c in Linux 5.9.14 source code
+ */
 static uint8_t TtyGetVgaAttr(struct Tty *tty)
 {
   uint8_t attr = tty->fg | tty->bg << 4;
-  if ((tty->pr & kTtyBold) != 0)
-    attr |= 0x08;
 #ifdef VGA_USE_BLINK
+  /*
+   * If blinking is enabled, we can only have the 8 dark background color
+   * codes (0 to 7).  Simply map any bright background color (8 to 15) to
+   * its corresponding dark color, & call it a day.  This is a bit more
+   * simplistic than what Linux does, but should be enough.
+   */
+  attr &= ~0x80;
   if ((tty->pr & kTtyBlink) != 0)
     attr |= 0x80;
 #endif
+  if ((tty->pr & kTtyBold) != 0)
+    attr |= 0x08;
   return attr;
 }
 
@@ -642,37 +656,23 @@ static void TtyCsiN(struct Tty *tty) {
 
 /**
  * Map the given (R, G, B) triplet to one of the 16 basic foreground colors
- * or one of the 8 (or 16) background colors.
+ * or one of the 16 background colors.
  *
- * @see VGA_USE_BLINK macro (libc/vga/vga.internal.h)
  * @see drivers/tty/vt/vt.c in Linux 5.9.14 source code
  */
-static uint8_t TtyMapTrueColor(uint8_t r, uint8_t g, uint8_t b, bool as_fg)
+static uint8_t TtyMapTrueColor(uint8_t r, uint8_t g, uint8_t b)
 {
-  uint8_t hue = 0;
-#ifdef VGA_USE_BLINK
-  if (!as_fg) {
-    if (r >= 128)
-      hue |= 4;
-    if (g >= 128)
-      hue |= 2;
-    if (b >= 128)
-      hue |= 1;
-  } else
-#endif
-  {
-    uint8_t max = MAX(MAX(r, g), b);
-    if (r > max / 2)
-      hue |= 4;
-    if (g > max / 2)
-      hue |= 2;
-    if (b > max / 2)
-      hue |= 1;
-    if (hue == 7 && max <= 0x55)
-      hue = 8;
-    else if (max > 0xaa)
-      hue |= 8;
-  }
+  uint8_t hue = 0, max = MAX(MAX(r, g), b);
+  if (r > max / 2)
+    hue |= 4;
+  if (g > max / 2)
+    hue |= 2;
+  if (b > max / 2)
+    hue |= 1;
+  if (hue == 7 && max <= 0x55)
+    hue = 8;
+  else if (max > 0xaa)
+    hue |= 8;
   return hue;
 }
 
@@ -682,7 +682,7 @@ static uint8_t TtyMapTrueColor(uint8_t r, uint8_t g, uint8_t b, bool as_fg)
  *
  * @see drivers/tty/vt/vt.c in Linux 5.9.14 source code
  */
-static uint8_t TtyMapXtermColor(uint8_t color, bool as_fg)
+static uint8_t TtyMapXtermColor(uint8_t color)
 {
   uint8_t r, g, b;
   if (color < 8) {
@@ -702,7 +702,7 @@ static uint8_t TtyMapXtermColor(uint8_t color, bool as_fg)
     r = color * 0x55 / 2;
   } else
     r = g = b = (unsigned)color * 10 - 2312;
-  return TtyMapTrueColor(r, g, b, as_fg);
+  return TtyMapTrueColor(r, g, b);
 }
 
 static void TtySelectGraphicsRendition(struct Tty *tty) {
@@ -825,13 +825,7 @@ static void TtySelectGraphicsRendition(struct Tty *tty) {
                 break;
               case 100 ... 107:
                 code[0] -= 100 - 40;
-#ifndef VGA_USE_BLINK
-                /*
-                 * If blinking is not enabled in VGA text mode, then we can
-                 * use bright background colors.
-                 */
                 code[0] += 8;
-#endif
                 /* fallthrough */
               case 40 ... 47:
                 tty->bg = code[0] - 40;
@@ -858,7 +852,7 @@ static void TtySelectGraphicsRendition(struct Tty *tty) {
             if (++code[3] == 3) {
               code[3] = 0;
               t = kSgr;
-              tty->fg = TtyMapTrueColor(code[0], code[1], code[2], true);
+              tty->fg = TtyMapTrueColor(code[0], code[1], code[2]);
               tty->pr |= kTtyFg;
               tty->pr |= kTtyTrue;
             }
@@ -867,20 +861,20 @@ static void TtySelectGraphicsRendition(struct Tty *tty) {
             if (++code[3] == 3) {
               code[3] = 0;
               t = kSgr;
-              tty->bg = TtyMapTrueColor(code[0], code[1], code[2], false);
+              tty->bg = TtyMapTrueColor(code[0], code[1], code[2]);
               tty->pr |= kTtyBg;
               tty->pr |= kTtyTrue;
             }
             break;
           case kSgrFgXterm:
             t = kSgr;
-            tty->fg = TtyMapXtermColor(code[0], true);
+            tty->fg = TtyMapXtermColor(code[0]);
             tty->pr |= kTtyFg;
             tty->pr &= ~kTtyTrue;
             break;
           case kSgrBgXterm:
             t = kSgr;
-            tty->bg = TtyMapXtermColor(code[0], false);
+            tty->bg = TtyMapXtermColor(code[0]);
             tty->pr |= kTtyBg;
             tty->pr &= ~kTtyTrue;
             break;