Add CLI option to set number of render threads

src/fart/Cli.cpp

@@ -11,6 +11,8 @@
     FLAG(help, "help", false) \
     FLAG(output, "o", true) \
     FLAG(output_l, "output", true) \
+    FLAG(threads, "t", true) \
+    FLAG(threads_l, "threads", true) \
     FLAG(version, "version", false) \
     FLAG(width, "w", true) \
     FLAG(width_l, "width", true) \
@@ -184,6 +186,7 @@ int Cli::handle_option(size_t flag_idx, const char * arg_value)
         printf("  -h, --height H        Set render height to H\n");
         printf("  --help                Show this help and exit\n");
         printf("  -o, --output F        Write rendered image to file F\n");
+        printf("  -t, --threads N       Use N render threads (default: number of CPUs)\n");
         printf("  -w, --width W         Set render width to W\n");
         printf("  --version             Show fart version and exit\n");
         return 1;
@@ -193,6 +196,19 @@ int Cli::handle_option(size_t flag_idx, const char * arg_value)
         this->output_file = arg_value;
         return 0;
 
+    case flag_threads:
+    case flag_threads_l:
+        if (!parse_size(flag_defs[flag_idx].flag, arg_value, &this->params.threads))
+        {
+            return 2;
+        }
+        if (*this->params.threads == 0u)
+        {
+            fprintf(stderr, "Error: option %s must be at least 1\n", flag_defs[flag_idx].flag);
+            return 2;
+        }
+        return 0;
+
     case flag_width:
     case flag_width_l:
         if (!parse_size(flag_defs[flag_idx].flag, arg_value, &this->params.width))

src/fart/RenderParams.cpp

@@ -1,4 +1,5 @@
 #include <fart/RenderParams.h>
+#include <thread>
 
 void RenderParams::merge(const RenderParams & other)
 {
@@ -10,6 +11,10 @@ void RenderParams::merge(const RenderParams & other)
     {
         height = other.height;
     }
+    if (!threads)
+    {
+        threads = other.threads;
+    }
 }
 
 void RenderParams::apply_defaults()
@@ -22,4 +27,10 @@ void RenderParams::apply_defaults()
     {
         height = 600u;
     }
+    if (!threads)
+    {
+        /* hardware_concurrency() may return 0 if it cannot be determined. */
+        size_t n = std::thread::hardware_concurrency();
+        threads = n > 0u ? n : 1u;
+    }
 }

src/fart/RenderParams.h

@@ -6,6 +6,7 @@ struct RenderParams
 {
     opt<size_t> width;
     opt<size_t> height;
+    opt<size_t> threads;
 
     /* Set each field that is not already set to the value from other. */
     void merge(const RenderParams & other);