Simplfy component ordering.

src/pixie/fileformats/jpeg.nim

@@ -344,7 +344,7 @@ proc decodeSOS(state: var DecoderState) =
   if state.scanComponents notin [1, 3]:
     failInvalid("unsupported scan component count")
 
-  state.componentOrder = @[]
+  state.componentOrder.setLen(0)
 
   for i in 0 ..< state.scanComponents:
     let
@@ -799,9 +799,9 @@ template checkReset(state: var DecoderState) =
 
 proc decodeBlocks(state: var DecoderState) =
   ## Decodes scan data blocks that follow a SOS block.
-  if state.progressive:
   if state.scanComponents == 1:
     # Single component pass.
+
     let
       comp = state.componentOrder[0]
       w = (state.components[comp].width + 7) shr 3
@@ -810,29 +810,17 @@ proc decodeBlocks(state: var DecoderState) =
       for row in 0 ..< w:
         state.decodeBlock(comp, row, column)
         state.checkReset()
-    else:
-      # Interleaved component pass.
-      for y in 0 ..< state.numMcuHigh:
-        for x in 0 ..< state.numMcuWide:
-          for comp in state.componentOrder:
-            for j in 0 ..< state.components[comp].yScale:
-              for i in 0 ..< state.components[comp].xScale:
-                let
-                  row = (x * state.components[comp].xScale + i)
-                  column = (y * state.components[comp].yScale + j)
-                state.decodeBlock(comp, row, column)
-          state.checkReset()
   else:
     # Interleaved regular component pass.
-    for y in 0 ..< state.numMcuHigh:
-      for x in 0 ..< state.numMcuWide:
+    for mcuY in 0 ..< state.numMcuHigh:
+      for mcuX in 0 ..< state.numMcuWide:
         for comp in state.componentOrder:
-          for j in 0 ..< state.components[comp].xScale:
-            for i in 0 ..< state.components[comp].yScale:
+          for compY in 0 ..< state.components[comp].xScale:
+            for compX in 0 ..< state.components[comp].yScale:
               let
-                row = (x * state.components[comp].yScale + i)
-                column = (y * state.components[comp].xScale + j)
-              state.decodeBlock(comp, row, column)
+                row = (mcuX * state.components[comp].yScale + compX)
+                col = (mcuY * state.components[comp].xScale + compY)
+              state.decodeBlock(comp, row, col)
         state.checkReset()
 
 proc quantizationAndIDCTPass(state: var DecoderState) =