I have only one thing to say in response, but it's a mouthful:
static int stbtt__run_charstring(const stbtt_fontinfo *info, int glyph_index, stbtt__csctx *c)
{
int in_header = 1, maskbits = 0, subr_stack_height = 0, sp = 0, v, i, b0;
int has_subrs = 0, clear_stack;
float s[48];
stbtt__buf subr_stack[10], subrs = info>subrs, b;
float f;
#define STBTT__CSERR(s) (0)
b = stbtt__cff_index_get(info>charstrings, glyph_index);
while (b.cursor < b.size) {
i = 0;
clear_stack = 1;
b0 = stbtt__buf_get8(&b);
switch (b0) {
case 0x13: case 0x14: if (in_header)
maskbits += (sp / 2); in_header = 0;
stbtt__buf_skip(&b, (maskbits + 7) / 8);
break;
case 0x01: case 0x03: case 0x12: case 0x17: maskbits += (sp / 2);
break;
case 0x15: in_header = 0;
if (sp < 2) return STBTT__CSERR("rmoveto stack");
stbtt__csctx_rmove_to(c, s[sp2], s[sp1]);
break;
case 0x04: in_header = 0;
if (sp < 1) return STBTT__CSERR("vmoveto stack");
stbtt__csctx_rmove_to(c, 0, s[sp1]);
break;
case 0x16: in_header = 0;
if (sp < 1) return STBTT__CSERR("hmoveto stack");
stbtt__csctx_rmove_to(c, s[sp1], 0);
break;
case 0x05: if (sp < 2) return STBTT__CSERR("rlineto stack");
for (; i + 1 < sp; i += 2)
stbtt__csctx_rline_to(c, s[i], s[i+1]);
break;
case 0x07: if (sp < 1) return STBTT__CSERR("vlineto stack");
goto vlineto;
case 0x06: if (sp < 1) return STBTT__CSERR("hlineto stack");
for (;;) {
if (i >= sp) break;
stbtt__csctx_rline_to(c, s[i], 0);
i++;
vlineto:
if (i >= sp) break;
stbtt__csctx_rline_to(c, 0, s[i]);
i++;
}
break;
case 0x1F: if (sp < 4) return STBTT__CSERR("hvcurveto stack");
goto hvcurveto;
case 0x1E: if (sp < 4) return STBTT__CSERR("vhcurveto stack");
for (;;) {
if (i + 3 >= sp) break;
stbtt__csctx_rccurve_to(c, 0, s[i], s[i+1], s[i+2], s[i+3], (sp  i == 5) ? s[i + 4] : 0.0f);
i += 4;
hvcurveto:
if (i + 3 >= sp) break;
stbtt__csctx_rccurve_to(c, s[i], 0, s[i+1], s[i+2], (sp  i == 5) ? s[i+4] : 0.0f, s[i+3]);
i += 4;
}
break;
case 0x08: if (sp < 6) return STBTT__CSERR("rcurveline stack");
for (; i + 5 < sp; i += 6)
stbtt__csctx_rccurve_to(c, s[i], s[i+1], s[i+2], s[i+3], s[i+4], s[i+5]);
break;
case 0x18: if (sp < 8) return STBTT__CSERR("rcurveline stack");
for (; i + 5 < sp  2; i += 6)
stbtt__csctx_rccurve_to(c, s[i], s[i+1], s[i+2], s[i+3], s[i+4], s[i+5]);
if (i + 1 >= sp) return STBTT__CSERR("rcurveline stack");
stbtt__csctx_rline_to(c, s[i], s[i+1]);
break;
case 0x19: if (sp < 8) return STBTT__CSERR("rlinecurve stack");
for (; i + 1 < sp  6; i += 2)
stbtt__csctx_rline_to(c, s[i], s[i+1]);
if (i + 5 >= sp) return STBTT__CSERR("rlinecurve stack");
stbtt__csctx_rccurve_to(c, s[i], s[i+1], s[i+2], s[i+3], s[i+4], s[i+5]);
break;
case 0x1A: case 0x1B: if (sp < 4) return STBTT__CSERR("(vvhh)curveto stack");
f = 0.0;
if (sp & 1) { f = s[i]; i++; }
for (; i + 3 < sp; i += 4) {
if (b0 == 0x1B)
stbtt__csctx_rccurve_to(c, s[i], f, s[i+1], s[i+2], s[i+3], 0.0);
else
stbtt__csctx_rccurve_to(c, f, s[i], s[i+1], s[i+2], 0.0, s[i+3]);
f = 0.0;
}
break;
case 0x0A: if (!has_subrs) {
if (info>fdselect.size)
subrs = stbtt__cid_get_glyph_subrs(info, glyph_index);
has_subrs = 1;
}
case 0x1D: if (sp < 1) return STBTT__CSERR("call(g)subr stack");
v = (int) s[sp];
if (subr_stack_height >= 10) return STBTT__CSERR("recursion limit");
subr_stack[subr_stack_height++] = b;
b = stbtt__get_subr(b0 == 0x0A ? subrs : info>gsubrs, v);
if (b.size == 0) return STBTT__CSERR("subr not found");
b.cursor = 0;
clear_stack = 0;
break;
case 0x0B: if (subr_stack_height <= 0) return STBTT__CSERR("return outside subr");
b = subr_stack[subr_stack_height];
clear_stack = 0;
break;
case 0x0E: stbtt__csctx_close_shape(c);
return 1;
case 0x0C: { float dx1, dx2, dx3, dx4, dx5, dx6, dy1, dy2, dy3, dy4, dy5, dy6;
float dx, dy;
int b1 = stbtt__buf_get8(&b);
switch (b1) {
case 0x22: if (sp < 7) return STBTT__CSERR("hflex stack");
dx1 = s[0];
dx2 = s[1];
dy2 = s[2];
dx3 = s[3];
dx4 = s[4];
dx5 = s[5];
dx6 = s[6];
stbtt__csctx_rccurve_to(c, dx1, 0, dx2, dy2, dx3, 0);
stbtt__csctx_rccurve_to(c, dx4, 0, dx5, dy2, dx6, 0);
break;
case 0x23: if (sp < 13) return STBTT__CSERR("flex stack");
dx1 = s[0];
dy1 = s[1];
dx2 = s[2];
dy2 = s[3];
dx3 = s[4];
dy3 = s[5];
dx4 = s[6];
dy4 = s[7];
dx5 = s[8];
dy5 = s[9];
dx6 = s[10];
dy6 = s[11];
stbtt__csctx_rccurve_to(c, dx1, dy1, dx2, dy2, dx3, dy3);
stbtt__csctx_rccurve_to(c, dx4, dy4, dx5, dy5, dx6, dy6);
break;
case 0x24: if (sp < 9) return STBTT__CSERR("hflex1 stack");
dx1 = s[0];
dy1 = s[1];
dx2 = s[2];
dy2 = s[3];
dx3 = s[4];
dx4 = s[5];
dx5 = s[6];
dy5 = s[7];
dx6 = s[8];
stbtt__csctx_rccurve_to(c, dx1, dy1, dx2, dy2, dx3, 0);
stbtt__csctx_rccurve_to(c, dx4, 0, dx5, dy5, dx6, (dy1+dy2+dy5));
break;
case 0x25: if (sp < 11) return STBTT__CSERR("flex1 stack");
dx1 = s[0];
dy1 = s[1];
dx2 = s[2];
dy2 = s[3];
dx3 = s[4];
dy3 = s[5];
dx4 = s[6];
dy4 = s[7];
dx5 = s[8];
dy5 = s[9];
dx6 = dy6 = s[10];
dx = dx1+dx2+dx3+dx4+dx5;
dy = dy1+dy2+dy3+dy4+dy5;
if (STBTT_fabs(dx) > STBTT_fabs(dy))
dy6 = dy;
else
dx6 = dx;
stbtt__csctx_rccurve_to(c, dx1, dy1, dx2, dy2, dx3, dy3);
stbtt__csctx_rccurve_to(c, dx4, dy4, dx5, dy5, dx6, dy6);
break;
default:
return STBTT__CSERR("unimplemented");
}
} break;
default:
if (b0 != 255 && b0 != 28 && (b0 < 32  b0 > 254))
return STBTT__CSERR("reserved operator");
if (b0 == 255) {
f = (float)(stbtt_int32)stbtt__buf_get32(&b) / 0x10000;
} else {
stbtt__buf_skip(&b, 1);
f = (float)(stbtt_int16)stbtt__cff_int(&b);
}
if (sp >= 48) return STBTT__CSERR("push stack overflow");
s[sp++] = f;
clear_stack = 0;
break;
}
if (clear_stack) sp = 0;
}
return STBTT__CSERR("no endchar");
#undef STBTT__CSERR
}
I laugh so I don't cry.
Real programmers use butterflies
