optimization: streamline image loading and remove pixel vector

- Read file once and decode from memory using stbi_load_from_memory
- Add guard for files smaller than 8 bytes
- Remove std::vector<Pixel> and accumulate histogram in downsample loop

main.cpp

@@ -7,6 +7,7 @@
 #include <vector>
 #include <unordered_map>
 #include <algorithm>
+#include <iterator>
 #include <cstdio>
 
 using std::cout;
@@ -15,10 +16,6 @@ namespace fs = std::filesystem;
 bool is_jpeg(const unsigned char* buf);
 bool is_png(const unsigned char* buf);
 
-struct Pixel {
-    unsigned char r, g, b;
-};
-
 struct Bucket {
     long r = 0;
     long g = 0;
@@ -62,22 +59,30 @@ int main(int argc, char *argv[]) {
         return 1;
     }
 
-    unsigned char header[8];
-    file.read(reinterpret_cast<char*>(header), 8);
+    // Read the file once; detect the format from the buffer and decode from
+    // memory so the image is never read from disk a second time.
+    std::vector<unsigned char> buffer((std::istreambuf_iterator<char>(file)),
+                                      std::istreambuf_iterator<char>());
+    if (buffer.size() < 8) {
+        cout << "File is too small to be a valid image!\n";
+        return 1;
+    }
+    const unsigned char* header = buffer.data();
 
     if (is_png(header)) {
         cout << "PNG file detected!\n";
     } else if (is_jpeg(header)) {
         cout << "JPEG file detected!\n";
     } else {
-        cout << "Unkown image file format!\n";
+        cout << "Unknown image file format!\n";
         return 1;
     }
 
     int width, height, channels;
 
-    unsigned char* data = stbi_load(
-        argv[1],
+    unsigned char* data = stbi_load_from_memory(
+        buffer.data(),
+        static_cast<int>(buffer.size()),
         &width,
         &height,
         &channels,
@@ -97,46 +102,42 @@ int main(int argc, char *argv[]) {
     cout << "Channels: " << channels << "\n";
     cout << "**********\n";
 
-    std::vector<Pixel> samples;
-
-    int step = 10;          //downsample rate
-    for (int y = 0; y< height; y += step) {
-        for (int x = 0; x < width; x+= step) {
-            int idx = (y * width + x) * 3;
-
-            Pixel p;
-            p.r = data[idx];
-            p.g = data[idx + 1];
-            p.b = data[idx + 2];
-            samples.push_back(p);
-        }
-    }
-
-    cout << "Sampled pixels: " << samples.size() << "\n";
-
     std::unordered_map<int, Bucket> hist;
 
-    for (auto& p : samples) {
-        int brightness = p.r + p.g + p.b;
+    int step = 10;          //downsample rate
+    long sampled = 0;
+    for (int y = 0; y < height; y += step) {
+        for (int x = 0; x < width; x += step) {
+            int idx = (y * width + x) * 3;
+
+            unsigned char r = data[idx];
+            unsigned char g = data[idx + 1];
+            unsigned char b = data[idx + 2];
+            sampled++;
+
+            int brightness = r + g + b;
             if (brightness < 60) continue;     //filtering out dark pixels
 
-        int minc = std::min({p.r, p.g, p.b});
-        int maxc = std::max({p.r, p.g, p.b});
+            int minc = std::min({r, g, b});
+            int maxc = std::max({r, g, b});
             if (maxc - minc < 10) continue;    //filtering out low saturation
                                                //(gray) colors
 
-        int rq = p.r / 16;
-        int gq = p.g / 16;
-        int bq = p.b / 16;
+            int rq = r / 16;
+            int gq = g / 16;
+            int bq = b / 16;
 
             int key = (rq << 8) | (gq << 4) | bq;
 
             auto& bucket = hist[key];
-        bucket.r += p.r;
-        bucket.g += p.g;
-        bucket.b += p.b;
-        bucket.count ++;
+            bucket.r += r;
+            bucket.g += g;
+            bucket.b += b;
+            bucket.count++;
         }
+    }
+
+    cout << "Sampled pixels: " << sampled << "\n";
 
     std::vector<Bucket> buckets;