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Optimisations for the feeble computer #10

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7414659
Optimisations for the feeble computer
colinleroy Aug 7, 2025
9fc0bcc
Drop mMinCode entirely.
colinleroy Aug 8, 2025
3870644
Don't decrement mMaxCode, and check for < instead of <=
colinleroy Aug 10, 2025
6c601ef
Preinit ZAG coeffs[1-63] to 0
colinleroy Aug 10, 2025
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179 changes: 91 additions & 88 deletions picojpeg.c
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Original file line number Diff line number Diff line change
Expand Up @@ -155,9 +155,9 @@ static int16 gLastDC[3];

typedef struct HuffTableT
{
uint16 mMinCode[16];
uint8 mGetMore[16];
uint16 mMaxCode[16];
uint8 mValPtr[16];
uint8 mValPtr[16];
} HuffTable;

// DC - 192
Expand Down Expand Up @@ -261,6 +261,24 @@ static PJPG_INLINE void stuffChar(uint8 i)
gInBufLeft++;
}
//------------------------------------------------------------------------------
static PJPG_INLINE void skipChars(uint16 n)
{
// skip characters fast
while (n > gInBufLeft)
{
n -= gInBufLeft;
gInBufOfs += gInBufLeft;
gInBufLeft = 0;
getChar();
n--;
if (!gInBufLeft)
return;
}

while (n--)
getChar();
}
//------------------------------------------------------------------------------
static PJPG_INLINE uint8 getOctet(uint8 FFCheck)
{
uint8 c = getChar();
Expand All @@ -283,7 +301,7 @@ static uint16 getBits(uint8 numBits, uint8 FFCheck)
{
uint8 origBits = numBits;
uint16 ret = gBitBuf;

if (numBits > 8)
{
numBits -= 8;
Expand Down Expand Up @@ -345,57 +363,28 @@ static PJPG_INLINE uint8 getBit(void)
return ret;
}
//------------------------------------------------------------------------------
static uint16 getExtendTest(uint8 i)
{
switch (i)
{
case 0: return 0;
case 1: return 0x0001;
case 2: return 0x0002;
case 3: return 0x0004;
case 4: return 0x0008;
case 5: return 0x0010;
case 6: return 0x0020;
case 7: return 0x0040;
case 8: return 0x0080;
case 9: return 0x0100;
case 10: return 0x0200;
case 11: return 0x0400;
case 12: return 0x0800;
case 13: return 0x1000;
case 14: return 0x2000;
case 15: return 0x4000;
default: return 0;
}
}
uint16 extendTests[] = {
0, 0x1, 0x2, 0x4, 0x8, 0x10, 0x20, 0x40,
0x80, 0x100, 0x200, 0x400, 0x800, 0x1000, 0x2000, 0x4000
};
//------------------------------------------------------------------------------
static int16 getExtendOffset(uint8 i)
{
switch (i)
{
case 0: return 0;
case 1: return ((-1)<<1) + 1;
case 2: return ((-1)<<2) + 1;
case 3: return ((-1)<<3) + 1;
case 4: return ((-1)<<4) + 1;
case 5: return ((-1)<<5) + 1;
case 6: return ((-1)<<6) + 1;
case 7: return ((-1)<<7) + 1;
case 8: return ((-1)<<8) + 1;
case 9: return ((-1)<<9) + 1;
case 10: return ((-1)<<10) + 1;
case 11: return ((-1)<<11) + 1;
case 12: return ((-1)<<12) + 1;
case 13: return ((-1)<<13) + 1;
case 14: return ((-1)<<14) + 1;
case 15: return ((-1)<<15) + 1;
default: return 0;
}
uint16 extendOffsets[] = {
((-1)<<0) + 1, ((-1)<<1) + 1, ((-1)<<2) + 1, ((-1)<<3) + 1, ((-1)<<4) + 1,
((-1)<<5) + 1, ((-1)<<6) + 1, ((-1)<<7) + 1, ((-1)<<8) + 1, ((-1)<<9) + 1,
((-1)<<10) + 1, ((-1)<<11) + 1, ((-1)<<12) + 1, ((-1)<<13) + 1, ((-1)<<14) + 1,
((-1)<<15) + 1
};
//------------------------------------------------------------------------------
static PJPG_INLINE int16 huffExtend(uint16 x, uint8 s)
{
return ((x < getExtendTest(s)) ? ((int16)x + getExtendOffset(s)) : (int16)x);
if (s < 16)
{
uint16 t = extendTests[s];
if (t > x)
return (int16)x + extendOffsets[s];
}

return (int16)x;
}
//------------------------------------------------------------------------------
static PJPG_INLINE uint8 huffDecode(const HuffTable* pHuffTable, const uint8* pHuffVal)
Expand All @@ -409,22 +398,21 @@ static PJPG_INLINE uint8 huffDecode(const HuffTable* pHuffTable, const uint8* pH
// more reasonable approach.
for ( ; ; )
{
uint16 maxCode;

if (i == 16)
return 0;

maxCode = pHuffTable->mMaxCode[i];
if ((code <= maxCode) && (maxCode != 0xFFFF))
break;
if (!pHuffTable->mGetMore[i])
{
if (code < pHuffTable->mMaxCode[i])
break;
}

i++;
code <<= 1;
code |= getBit();
}

j = pHuffTable->mValPtr[i];
j = (uint8)(j + (code - pHuffTable->mMinCode[i]));
j = (uint8)(pHuffTable->mValPtr[i] + code);

return pHuffVal[j];
}
Expand All @@ -440,22 +428,17 @@ static void huffCreate(const uint8* pBits, HuffTable* pHuffTable)
{
uint8 num = pBits[i];

if (!num)
{
pHuffTable->mMinCode[i] = 0x0000;
pHuffTable->mMaxCode[i] = 0xFFFF;
pHuffTable->mValPtr[i] = 0;
}
else
if (num)
{
pHuffTable->mMinCode[i] = code;
pHuffTable->mMaxCode[i] = code + num - 1;
pHuffTable->mValPtr[i] = j;
pHuffTable->mMaxCode[i] = code + num;
pHuffTable->mValPtr[i] = j - (uint8)code;

j = (uint8)(j + num);

code = (uint16)(code + num);
}
else
pHuffTable->mGetMore[i] = 1;

code <<= 1;

Expand Down Expand Up @@ -653,18 +636,32 @@ static uint8 readSOFMarker(void)
static uint8 skipVariableMarker(void)
{
uint16 left = getBits1(16);
uint16 safeSkip;

if (left < 2)
return PJPG_BAD_VARIABLE_MARKER;
switch (left)
{
case 0:
case 1:
return PJPG_BAD_VARIABLE_MARKER;
case 2:
return 0;
case 3:
getBits1(8);
return 0;
default:
safeSkip = left - 4;
left = 2;
}

left -= 2;
// Avoid shifting gBitBuf for all but the last
// two bytes that we want to skip.
skipChars(safeSkip);

while (left)
while(left--)
{
getBits1(8);
left--;
getBits1(8);
}

return 0;
}
//------------------------------------------------------------------------------
Expand Down Expand Up @@ -2140,7 +2137,12 @@ static uint8 decodeNextMCU(void)
const int16* pQ = compQuant ? gQuant1 : gQuant0;
uint16 r, dc;

uint8 s = huffDecode(compDCTab ? &gHuffTab1 : &gHuffTab0, compDCTab ? gHuffVal1 : gHuffVal0);
uint8 s;

if (compDCTab)
s = huffDecode(&gHuffTab1, gHuffVal1);
else
s = huffDecode(&gHuffTab0, gHuffVal0);

r = 0;
numExtraBits = s & 0xF;
Expand All @@ -2153,14 +2155,22 @@ static uint8 decodeNextMCU(void)

gCoeffBuf[0] = dc * pQ[0];

/* Pre-zero the other coeffs */
for (k = 1; k < 64; k++) {
gCoeffBuf[k] = 0;
}

compACTab = gCompACTab[componentID];

if (gReduce)
{
// Decode, but throw out the AC coefficients in reduce mode.
for (k = 1; k < 64; k++)
{
s = huffDecode(compACTab ? &gHuffTab3 : &gHuffTab2, compACTab ? gHuffVal3 : gHuffVal2);
if (compACTab)
s = huffDecode(&gHuffTab3, gHuffVal3);
else
s = huffDecode(&gHuffTab2, gHuffVal2);

numExtraBits = s & 0xF;
if (numExtraBits)
Expand Down Expand Up @@ -2202,7 +2212,10 @@ static uint8 decodeNextMCU(void)
{
uint16 extraBits;

s = huffDecode(compACTab ? &gHuffTab3 : &gHuffTab2, compACTab ? gHuffVal3 : gHuffVal2);
if (compACTab)
s = huffDecode(&gHuffTab3, gHuffVal3);
else
s = huffDecode(&gHuffTab2, gHuffVal2);

extraBits = 0;
numExtraBits = s & 0xF;
Expand All @@ -2221,11 +2234,7 @@ static uint8 decodeNextMCU(void)
if ((k + r) > 63)
return PJPG_DECODE_ERROR;

while (r)
{
gCoeffBuf[ZAG[k++]] = 0;
r--;
}
k = (uint8)(k + r);
}

ac = huffExtend(extraBits, s);
Expand All @@ -2239,18 +2248,12 @@ static uint8 decodeNextMCU(void)
if ((k + 16) > 64)
return PJPG_DECODE_ERROR;

for (r = 16; r > 0; r--)
gCoeffBuf[ZAG[k++]] = 0;

k--; // - 1 because the loop counter is k
k += (16 - 1); // - 1 because the loop counter is k
}
else
break;
}
}

while (k < 64)
gCoeffBuf[ZAG[k++]] = 0;

transformBlock(mcuBlock);
}
Expand Down