From 74dc73ea31f2732cd77d6254ee0e372e97261693 Mon Sep 17 00:00:00 2001
From: quou <quou@disroot.org>
Date: Sun, 28 Jul 2024 16:34:42 +1000
Subject: Add triangle sample

---
 tri.c | 114 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 114 insertions(+)
 create mode 100644 tri.c

diff --git a/tri.c b/tri.c
new file mode 100644
index 0000000..e8f0e24
--- /dev/null
+++ b/tri.c
@@ -0,0 +1,114 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+typedef struct {
+	int x, y;
+	int dx, dy;
+	int sx, sy;
+	int e;
+} Line;
+
+void init_line(
+	Line* l,
+	const int* f,
+	const int* t
+) {
+	const int dx = abs(t[0] - f[0]);
+	const int dy = -abs(t[1] - f[1]);
+	l->e = dx + dy;
+	l->dx = dx;
+	l->dy = dy;
+	l->sx = f[0] < t[0]? 1: -1;
+	l->sy = f[1] < t[1]? 1: -1;
+	l->x = f[0];
+	l->y = f[1];
+}
+
+void step_line(Line* l) {
+	const int e2 = l->e * 2;
+	const int dx = l->dx;
+	const int dy = l->dy;
+	if (e2 > dy) {
+		l->e += dy;
+		l->x += l->sx;
+	}
+	if (e2 <= dx) {
+		l->e += dx;
+		l->y += l->sy;
+	}
+}
+
+int tri_hand(
+	const int* v0,
+	const int* v1,
+	const int* v2
+) {
+	return
+		(v1[0] - v0[0]) * (v2[1] - v0[1]) -
+		(v2[0] - v0[0]) * (v1[1] - v0[1]) > 0;
+}
+
+void fill_tri(
+	char* buf, int w,
+	const int* v0,
+	const int* v1,
+	const int* v2
+) {
+	Line ne, fe;
+	const int* t;
+	int ha, x;
+#define swap(a, b) { \
+	t = a; \
+	a = b; \
+	b = t; \
+}
+	if (v0[1] > v1[1])
+		swap(v0, v1);
+	if (v1[1] > v2[1])
+		swap(v1, v2);
+	if (v0[1] > v1[1])
+		swap(v0, v1);
+#undef swap
+	ha = tri_hand(v0, v1, v2);
+	if (ha) {
+		init_line(&ne, v0, v2);
+		init_line(&fe, v0, v1);
+	} else {
+		init_line(&ne, v0, v1);
+		init_line(&fe, v0, v2);
+	}
+#define scanline \
+	for (x = ne.x; x < fe.x; x++) \
+		buf[x + fe.y * w] = '#'; \
+	step_line(&fe); \
+	while (ne.y != fe.y) \
+		step_line(&ne);
+	while (fe.y < v1[1]) {
+		scanline
+	}
+	if (ha)
+		init_line(&fe, v1, v2);
+	else
+		init_line(&ne, v1, v2);
+	while (fe.y < v2[1]) {
+		scanline
+	}
+#undef scanline
+}
+
+int main(void) {
+	int v0[] = { 16, 2  };
+	int v1[] = { 31, 16 };
+	int v2[] = { 1,  30 };
+	char buf[32*32];
+	int i;
+	for (i = 0; i < 32*32; i++)
+		buf[i] = ' ';
+	fill_tri(buf, 32, v0, v1, v2);
+	for (i = 0; i < 32*32; i++) {
+		if (!(i & 31)) printf("\n");
+		printf("%c", buf[i]);
+	}
+	printf("\n");
+}
+
-- 
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