fonts: speed up/simplify fetching glyph data
What the developer wrote
Authored by Jon Griffiths
fonts: speed up/simplify fetching glyph data
Note that glyph looking is still O(N), and thus string length
calculation and display is O(N^2). This commit merely makes the existing
linear lookup faster.
The linear lookup is a consequence of the font structure allowing
characters to be out of order. If we were to force the characters to be
ordered in the font definition we could trivially implement an index
to allow even faster lookups.
What changed, and why it matters
This commit is a straightforward internal cleanup of how a hardware wallet's screen font lookup works. It rewrites a helper function to be faster and simpler, but does not change what the function does or fix any security problem. There is no indication this is a security patch.
No security action needed. Treat as a normal performance/refactoring commit.
Security signals we found
No strong security signals were identified.
Evidence from the diff
The change refactors get_char_ptr() in main/display.c. The old implementation walked the font table using a 16-bit index and copied fields into a global static propFont (renamed PropFont). The new version uses a direct byte pointer, skips glyph bitmap data with the same arithmetic, and writes results into a caller-supplied PropFont* output parameter. The lookup remains linear O(N) as noted in the commit message. No bounds checks, input validation, or security-relevant behavior is altered; the function still terminates on charCode 0xFF and returns 0 if the character is not found.
Changed components
main/display.c font glyph lookup helperInspect captured patch +23 / −29
diff --git a/main/display.c b/main/display.c
index c087533..c5f6ff2 100644
--- a/main/display.c
+++ b/main/display.c
@@ -451,9 +451,9 @@ typedef struct {
int xOffset;
int xDelta;
uint16_t dataPtr;
-} propFont;
+} PropFont;
-static propFont fontChar;
+static PropFont fontChar;
#ifdef CONFIG_DISPLAY_FULL_FRAME_BUFFER
uint16_t* get_display_buffer_at(int x, int y)
@@ -612,35 +612,29 @@ static int print_proportional_char(int x, int y)
return char_width;
}
-static uint8_t get_char_ptr(const uint8_t c)
+static uint8_t get_char_ptr(const uint8_t c, PropFont* char_out)
{
- uint16_t tempPtr = 4;
-
- do {
- fontChar.charCode = cfont.font[tempPtr++];
- if (fontChar.charCode == 0xFF) {
- return 0;
- }
-
- fontChar.adjYOffset = cfont.font[tempPtr++];
- fontChar.width = cfont.font[tempPtr++];
- fontChar.height = cfont.font[tempPtr++];
- fontChar.xOffset = cfont.font[tempPtr++];
- fontChar.xOffset = fontChar.xOffset < 0x80 ? fontChar.xOffset : -(0xFF - fontChar.xOffset);
- fontChar.xDelta = cfont.font[tempPtr++];
+ const uint8_t* font_data = cfont.font + 4;
- if (c != fontChar.charCode && fontChar.charCode != 0xFF) {
- if (fontChar.width != 0) {
- tempPtr += (((fontChar.width * fontChar.height) - 1) / 8) + 1;
- }
+ while (*font_data != c && *font_data != 0xff) {
+ if (font_data[2]) {
+ // Non-zero width character. Skip packed glyph bitmap data.
+ font_data += (((int)font_data[2] * font_data[3] - 1) / 8) + 1;
}
- } while ((c != fontChar.charCode) && (fontChar.charCode != 0xFF));
-
- fontChar.dataPtr = tempPtr;
- if (c != fontChar.charCode) {
- return 0;
+ font_data += 6;
}
-
+ if (*font_data == 0xff) {
+ return 0; // Not found
+ }
+ // Found, copy data
+ char_out->charCode = *font_data++;
+ char_out->adjYOffset = *font_data++;
+ char_out->width = *font_data++;
+ char_out->height = *font_data++;
+ char_out->xOffset = *font_data++;
+ char_out->xOffset = fontChar.xOffset < 0x80 ? fontChar.xOffset : -(0xFF - fontChar.xOffset);
+ char_out->xDelta = *font_data++;
+ char_out->dataPtr = font_data - cfont.font;
return 1;
}
@@ -680,7 +674,7 @@ int display_get_string_width(const char* str)
int charWidth, xDelta;
while (*tempStrptr) {
- if (get_char_ptr(*tempStrptr++)) {
+ if (get_char_ptr(*tempStrptr++, &fontChar)) {
charWidth = fontChar.width;
xDelta = fontChar.xDelta;
strWidth += ((charWidth > xDelta) ? charWidth : xDelta) + 1;
@@ -884,7 +878,7 @@ void display_print_in_area(const char* st, int x, int y, dispWin_t areaWin, bool
}
} else {
if (!cfont.x_size) {
- if (get_char_ptr(ch)) {
+ if (get_char_ptr(ch, &fontChar)) {
tmpw = fontChar.xDelta;
} else {
continue;
Why this scored 15/100
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