feat: Output ordering!

As results come from the dispatcher('s return channel) they are pushed
into a vector to be reordered. They're sorted in reverse-order so that
they can be popped from the vector. Upon receipt and buffering of a
scanline, a loop checks the tail of the buffer to see if it's the
next-to-write element. Since the tail is popped off, this loop can run
until this condition is not met.

src/main.rs

@@ -66,7 +66,7 @@ fn main() {
     };
 
     thread::scope(|s| {
-        let (mut dispatcher, mut scanline_receiver) = thread_utils::Dispatcher::new(&small_rng);
+        let (mut dispatcher, scanline_receiver) = thread_utils::Dispatcher::new(&small_rng);
 
         s.spawn(move || {
             for y in (0..image.1).rev() {
@@ -83,14 +83,52 @@ fn main() {
             dispatcher.submit_job(RenderCommand::Stop);
             // ... also I happen to know there are 4 threads.
         });
-        eprintln!("Reached the scanline collector");
+
+        /*
+         * Store received results in the segments buffer.
+         * Some will land before their previous segments and will need to be held
+         * until the next-to-write arrives.
+         *
+         * Elements are sorted in reverse order so that they can be popped from the
+         * Vec quickly.
+         *
+         * The queue is scanned every single time a new item is received. In the
+         * happy path where the received item is next-up, it'll be buffered, checked
+         * and then printed. In the case where it isn't, it'll get buffered and
+         * stick around for more loops. When the next-to-write finally lands, it
+         * means the n+1 element is up, now. If that element is already in the buffer
+         * we want to write it out. Hence the loop that scans the whole buffer each
+         * receive.
+         *
+         * TODO: There could be an up-front conditional that checks to see if the
+         * received item *is* the next-to-write and skip the buffering step.
+         * But I need to make the concept work at all, first.
+         */
+        let mut raster_segments = Vec::<thread_utils::RenderResult>::new();
+        let mut sl_output_index = image.1-1; // scanlines count down, start at image height.
         while let Ok(scanline) = scanline_receiver.recv() {
-            //TODO: sort results once multiple threads are introduced.
             eprintln!("Received scanline: {}", scanline.line_num);
-            for color in scanline.line {
-                println!("{}", color.print_ppm(samples_per_pixel));
+
+            raster_segments.push(scanline);
+            raster_segments.sort_by( |a, b| b.cmp(a) );
+
+            loop {
+                if raster_segments.len() == 0 { break; } // can this ever happen? Not while every
+                                                         // single element gets pushed to the
+                                                         // buffer first. With the happy path
+                                                         // short-circuit noted above, it could.
+
+                let last_ind = raster_segments.len() - 1;
+                if raster_segments[last_ind].line_num == sl_output_index{
+                    let scanline = raster_segments.pop().unwrap();
+                    print_scanline(scanline, samples_per_pixel);
+                    sl_output_index -= 1;
+                } else {
+                    break;
+                }
             }
         }
+        eprintln!("Size of raster_segments at finish: {}", raster_segments.len());
     });
     
     
@@ -98,6 +136,13 @@ fn main() {
     eprintln!("Done!");
 }
 
+fn print_scanline(scanline: thread_utils::RenderResult, samples_per_pixel: u32){
+    eprintln!("Printing scanline num: {}", scanline.line_num);
+    for color in &scanline.line {
+        println!("{}", color.print_ppm(samples_per_pixel));
+    }
+}
+
 #[derive (Clone)]
 pub struct RenderContext{
     image: (i32, i32),

src/thread_utils.rs

@@ -3,6 +3,7 @@ use crate::RenderContext;
 use crate::Vec3;
 use crate::render_line;
 
+use core::cmp::Ordering;
 use std::thread;
 use std::sync::mpsc;
 use rand::rngs::SmallRng;
@@ -17,6 +18,32 @@ pub struct RenderResult {
     pub line: Vec<Vec3>,
 }
 
+impl Ord for RenderResult {
+    fn cmp(&self, other: &Self) -> Ordering {
+        if self.line_num > other.line_num {
+            Ordering::Less
+        } else if self.line_num < other.line_num {
+            Ordering::Greater
+        } else {
+            Ordering::Equal
+        }
+    }
+}
+
+impl PartialOrd for RenderResult {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+impl PartialEq for RenderResult {
+    fn eq(&self, other: &Self) -> bool {
+        self.line_num == other.line_num
+    }
+}
+
+impl Eq for RenderResult {}
+
 /*
  *  The dispatcher will hold a list of threads, and a list of command input channels to match.
  *  Helper functions exist to input jobs serially, and then dispatch them to an open thread.