From c24cd753604fa73c8c77cec9886f6a6835b4a03d Mon Sep 17 00:00:00 2001
From: Camden Dixie O'Brien <cdo@wip.sh>
Date: Sat, 28 Dec 2024 18:57:29 +0000
Subject: [PATCH] Implement terrain collisions

---
 app/main.c | 56 ++++++++++++++++++++++++++++++++++++++++++++----------
 1 file changed, 46 insertions(+), 10 deletions(-)

diff --git a/app/main.c b/app/main.c
index cc6cf93..ae8e8c3 100644
--- a/app/main.c
+++ b/app/main.c
@@ -34,11 +34,17 @@
 #define PWIDTH 48
 #define PHEIGHT 64
 #define PANIMLEN 8
+#define PFBWIDTH 12
+#define PFBHEIGHT 6
+#define PFBOFFX -6
+#define PFBOFFY 6
 
 #define WALKSPEED 72 // pixels per second
 
 #define BASEANIMPERIOD 100
 
+#define NELEMS(a) (sizeof(a) / sizeof(a[0]))
+
 typedef enum {
 	SPRITE_DIR_DOWN = 0,
 	SPRITE_DIR_LEFT_DOWN = 1,
@@ -61,12 +67,34 @@ static SDL_Renderer *renderer;
 static unsigned map[MAPWIDTH][MAPHEIGHT];
 static SDL_Texture *tstex, *pidle, *pwalk, *ptex;
 static input_state_t input;
+static dvec_t vpos = { 512, 0 };
+static const unsigned passable[] = { 118, 178 };
 
 static inline double mag(dvec_t v)
 {
 	return sqrt(v.x * v.x + v.y * v.y);
 }
 
+static inline unsigned tileat(double x, double y)
+{
+	const unsigned row = (unsigned)floor(x / TILESIZE) + MAPSHIFTX;
+	const unsigned col = (unsigned)floor(y / TILESIZE) + MAPSHIFTY;
+	if (row >= MAPWIDTH || col >= MAPHEIGHT)
+		return 0;
+	else
+		return map[row][col];
+}
+
+static inline bool tilepassable(int x, int y)
+{
+	const unsigned id = tileat(x, y);
+	for (unsigned i = 0; i < NELEMS(passable); ++i) {
+		if (passable[i] == id)
+			return true;
+	}
+	return false;
+}
+
 int main(int argc, char *argv[])
 {
 	char path[MAX_PATH_LEN];
@@ -176,8 +204,7 @@ int main(int argc, char *argv[])
 	pwalk = IMG_LoadTexture(renderer, path);
 	assert(NULL != pwalk);
 
-	// Initialize view and player
-	dvec_t vpos = { 512, 0 };
+	// Initialize player
 	SDL_Rect psrc = { .y = 0, .w = PWIDTH, .h = PHEIGHT };
 	SDL_Rect pdest = { .w = SCALE * PWIDTH, .h = SCALE * PHEIGHT };
 	dvec_t pvel = { 0, 0 }, ppos = { 640, 96 };
@@ -247,8 +274,21 @@ int main(int argc, char *argv[])
 		if (pspeed != 0) {
 			pvel.x *= WALKSPEED / pspeed;
 			pvel.y *= WALKSPEED / pspeed;
-			ppos.x += (double)dt / 1000.0 * pvel.x;
-			ppos.y += (double)dt / 1000.0 * pvel.y;
+
+			// Check for terrain collision, update position if none
+			const double nextx = ppos.x + (double)dt / 1000.0 * pvel.x;
+			const double nexty = ppos.y + (double)dt / 1000.0 * pvel.y;
+			const dvec_t pfb
+			    = { .x = nextx + PFBOFFX, .y = nexty + PFBOFFY };
+			bool valid = true;
+			valid &= tilepassable(pfb.x, pfb.y);
+			valid &= tilepassable(pfb.x + PFBWIDTH, pfb.y);
+			valid &= tilepassable(pfb.x, pfb.y + PFBHEIGHT);
+			valid &= tilepassable(pfb.x + PFBWIDTH, pfb.y + PFBHEIGHT);
+			if (valid) {
+				ppos.x = nextx;
+				ppos.y = nexty;
+			}
 
 			// Update direction
 			if (pvel.y >= 0) {
@@ -267,7 +307,6 @@ int main(int argc, char *argv[])
 					pdir = SPRITE_DIR_UP;
 			}
 
-			// Set walkikng texture
 			ptex = pwalk;
 		} else {
 			ptex = pidle;
@@ -290,11 +329,7 @@ int main(int argc, char *argv[])
 		const int starty = TILESIZE * floor(vpos.y / TILESIZE);
 		for (int y = starty; y < vpos.y + VIEWHEIGHT; y += TILESIZE) {
 			for (int x = startx; x < vpos.x + VIEWWIDTH; x += TILESIZE) {
-				const unsigned row = x / TILESIZE + MAPSHIFTX;
-				const unsigned col = y / TILESIZE + MAPSHIFTY;
-				if (row >= MAPWIDTH || col >= MAPHEIGHT)
-					continue;
-				const unsigned tileid = map[row][col];
+				const unsigned tileid = tileat(x, y);
 				if (0 == tileid)
 					continue;
 				const SDL_Rect src = {
@@ -320,6 +355,7 @@ int main(int argc, char *argv[])
 		pdest.x = SCALE * (int)(ppos.x - vpos.x - PWIDTH / 2);
 		pdest.y = SCALE * (int)(ppos.y - vpos.y - PHEIGHT / 2);
 		SDL_RenderCopy(renderer, ptex, &psrc, &pdest);
+
 		SDL_RenderPresent(renderer);
 	}