add rudimentary PPM decoder

src/pixie.nim

@@ -1,7 +1,8 @@
 import bumpy, chroma, flatty/binny, os, pixie/common, pixie/contexts,
     pixie/fileformats/bmp, pixie/fileformats/gif, pixie/fileformats/jpg,
-    pixie/fileformats/png, pixie/fileformats/ppm, pixie/fileformats/qoi, pixie/fileformats/svg,
-    pixie/fonts, pixie/images, pixie/masks, pixie/paints, pixie/paths, strutils, vmath
+    pixie/fileformats/png, pixie/fileformats/ppm, pixie/fileformats/qoi,
+    pixie/fileformats/svg, pixie/fonts, pixie/images, pixie/masks, pixie/paints,
+    pixie/paths, strutils, vmath
 
 export bumpy, chroma, common, contexts, fonts, images, masks, paints, paths, vmath
 

src/pixie/fileformats/ppm.nim

@@ -1,12 +1,99 @@
-import chroma, flatty/binny, pixie/common, pixie/images
+import chroma, flatty/binny, pixie/common, pixie/images, std/strutils
 
 # See: http://netpbm.sourceforge.net/doc/ppm.html
 
 const ppmSignatures* = @["P3", "P6"]
 
+type
+  PpmHeader = object
+    version: string
+    width, height, maxVal, dataOffset: int
+
+template failInvalid() =
+  raise newException(PixieError, "Invalid PPM data")
+
+proc decodeHeader(data: string): PpmHeader {.raises: [PixieError].} =
+  if data.len <= 10: # Each part + whitespace
+    raise newException(PixieError, "Invalid PPM file header")
+
+  var commentMode: bool
+  var i, readFields: int
+  var field: string
+  while readFields < 4:
+    let c = readUint8(data, i).char
+    if c == '#':
+      commentMode = true
+    elif c == '\n':
+      commentMode = false
+    if not commentMode:
+      if c in Whitespace:
+        readFields += 1
+        try:
+          case readFields:
+            of 1:
+              result.version = field
+            of 2:
+              result.width = parseInt(field)
+            of 3:
+              result.height = parseInt(field)
+            of 4:
+              result.maxVal = parseInt(field)
+            else:
+              discard
+        except ValueError: failInvalid()
+        field = ""
+      else:
+        field.add(c)
+    i += 1
+
+    result.dataOffset = i
+
+proc decodeP6Data(data: string, maxVal: int): seq[ColorRGBX] {.raises: [].} =
+  let needsUint16 = maxVal > 0xFF
+
+  result = newSeq[ColorRGBX]((
+    if needsUint16: data.len / 6
+    else: data.len / 3
+  ).int)
+
+  # Let's calculate the real maximum value multiplier.
+  # rgbx() accepts a maximum value of 0xFF. Most of the time,
+  # maxVal is set to 0xFF as well, so in most cases it is 1
+  let valueMultiplier = (0xFF / maxVal).uint8
+
+  # if comparison in for loops is expensive, so let's unroll it
+  if not needsUint16:
+    for i in 0 ..< result.len:
+      let
+        red = readUint8(data, i + (i * 2)) * valueMultiplier
+        green = readUint8(data, i + 1 + (i * 2)) * valueMultiplier
+        blue = readUint8(data, i + 2 + (i * 2)) * valueMultiplier
+      result[i] = rgbx(red, green, blue, 0xFF)
+  else:
+    for i in 0 ..< result.len:
+      let
+        red = readUint16(data, i + (i * 4)).uint8 * valueMultiplier
+        green = readUint16(data, i + 2 + (i * 4)).uint8 * valueMultiplier
+        blue = readUint16(data, i + 4 + (i * 4)).uint8 * valueMultiplier
+      result[i] = rgbx(red, green, blue, 0xFF)
+
+proc decodeP3Data(data: string, maxVal: int): seq[ColorRGBX] {.raises: [].} =
+  result = newSeq[ColorRGBX](1)
+
 proc decodePpm*(data: string): Image {.raises: [PixieError].} =
   ## Decodes Portable Pixel Map data into an Image.
-  result = newImage(0, 0)
+
+  let header = decodeHeader(data)
+
+  if not (header.version in ppmSignatures): failInvalid()
+  if 0 > header.maxVal or header.maxVal > 0xFFFF: failInvalid()
+
+  result = newImage(header.width, header.height)
+  result.data = (
+    if header.version == "P3":
+      decodeP3Data(data[header.dataOffset .. ^1], header.maxVal)
+    else: decodeP6Data(data[header.dataOffset .. ^1], header.maxVal)
+  )
 
 proc decodePpm*(data: seq[uint8]): Image {.inline, raises: [PixieError].} =
   ## Decodes Portable Pixel Map data into an Image.