Report render progress if stdout is a TTY

src/fart/Engine.cpp

@@ -1,8 +1,81 @@
 #include <fart/Engine.h>
 #include <fart/Parser.h>
+#include <chrono>
 #include <errno.h>
 #include <stdio.h>
 #include <string.h>
+#include <unistd.h>
+
+namespace
+{
+
+/**
+ * Reports render progress to stdout when stdout is a TTY.
+ *
+ * An update is written only when progress has increased by at least 1%,
+ * or when progress has increased at all and at least 1 second has elapsed
+ * since the last update.
+ */
+class ProgressReporter
+{
+public:
+    using clock = std::chrono::steady_clock;
+
+    explicit ProgressReporter(size_t total)
+        : total(total)
+        , enabled(total > 0u && isatty(STDOUT_FILENO))
+    {
+        if (enabled)
+        {
+            last_time = clock::now();
+            print(0u);
+        }
+    }
+
+    void update(size_t done)
+    {
+        if (!enabled || done <= last_done)
+        {
+            return;
+        }
+        /* (done - last_done) / total >= 1 / 100 */
+        bool percent_step = (done - last_done) * 100u >= total;
+        clock::time_point now = clock::now();
+        if (percent_step || (now - last_time) >= std::chrono::seconds(1))
+        {
+            last_time = now;
+            print(done);
+        }
+    }
+
+    void finish()
+    {
+        if (enabled)
+        {
+            if (last_done < total)
+            {
+                print(total);
+            }
+            fputc('\n', stdout);
+            fflush(stdout);
+        }
+    }
+
+protected:
+    size_t total;
+    bool enabled;
+    size_t last_done = 0u;
+    clock::time_point last_time;
+
+    void print(size_t done)
+    {
+        last_done = done;
+        printf("\rRendering: %6.2f%%", (double)done * 100.0 / (double)total);
+        fflush(stdout);
+    }
+};
+
+}
 
 Engine::Engine(const Options & options)
     : options(options)
@@ -67,13 +140,18 @@ int Engine::run(string_view input_file, opt<string_view> output_file)
     options.apply_defaults();
 
     Image image(*options.width, *options.height);
+    ProgressReporter progress(*options.width * *options.height);
+    size_t pixels_done = 0u;
     for (size_t y = 0u; y < *options.height; y++)
     {
         for (size_t x = 0u; x < *options.width; x++)
         {
-            image[x, y] = *options.background;
+            image[x, y] = render_pixel(x, y);
+            pixels_done++;
+            progress.update(pixels_done);
         }
     }
+    progress.finish();
 
     if (!image_writer->write(image))
     {
@@ -82,3 +160,8 @@ int Engine::run(string_view input_file, opt<string_view> output_file)
 
     return 0;
 }
+
+Color Engine::render_pixel(size_t x, size_t y)
+{
+    return *this->options.background;
+}

src/fart/Engine.h

@@ -16,4 +16,6 @@ protected:
     Options options;
     std::unique_ptr<ImageWriter> image_writer;
     Scene * scene = nullptr;
+
+    Color render_pixel(size_t x, size_t y);
 };