examples: move the particle module in modules/particle

The goal is enabling running `v build-examples` from outside vroot.
The modules/ folders are already skipped when building examples.

examples/sokol/particles/modules/particle/system.v

@@ -0,0 +1,101 @@
+// Copyright(C) 2019 Lars Pontoppidan. All rights reserved.
+// Use of this source code is governed by an MIT license file distributed with this software package
+module particle
+
+import vec2
+import rand
+import sokol.sgl
+
+pub struct SystemConfig {
+	pool int
+}
+
+pub struct System {
+	width  int
+	height int
+mut:
+	pool   []&Particle
+	bin    []&Particle
+}
+
+pub fn (mut s System) init(sc SystemConfig) {
+	for i := 0; i < sc.pool; i++ {
+		p := new(vec2.Vec2{f32(s.width) * 0.5, f32(s.height) * 0.5})
+		s.pool << p
+	}
+}
+
+pub fn (mut s System) update(dt f64) {
+	mut p := &Particle(0)
+	for i := 0; i < s.pool.len; i++ {
+		p = s.pool[i]
+		p.update(dt)
+		if p.is_dead() {
+			s.bin << p
+			s.pool.delete(i)
+		}
+	}
+}
+
+pub fn (s System) draw() {
+	sgl.begin_quads()
+	for p in s.pool {
+		p.draw()
+	}
+	sgl.end()
+}
+
+pub fn (mut s System) reset() {
+	for i in 0..s.pool.len {
+		mut p := s.pool[i]
+		p.reset()
+		p.life_time = 0
+	}
+	for i in 0..s.bin.len {
+		mut p := s.pool[i]
+		p.reset()
+		p.life_time = 0
+	}
+}
+
+pub fn (mut s System) explode(x, y f32) {
+	mut reserve := 500
+	center := vec2.Vec2{x, y}
+	mut p := &Particle(0)
+	for i := 0; i < s.bin.len && reserve > 0; i++ {
+		p = s.bin[i]
+		p.reset()
+		p.location.from(center)
+		p.acceleration = vec2.Vec2{rand.f32_in_range(-0.5, 0.5), rand.f32_in_range(-0.5,
+			0.5)}
+		p.velocity = vec2.Vec2{rand.f32_in_range(-0.5, 0.5), rand.f32_in_range(-0.5, 0.5)}
+		p.life_time = rand.f64_in_range(500, 2000)
+		s.pool << p
+		s.bin.delete(i)
+		reserve--
+	}
+}
+
+pub fn (mut s System) free() {
+	for p in s.pool {
+		if p == 0 {
+			print(ptr_str(p) + ' ouch')
+			continue
+		}
+		unsafe {
+			free(p)
+		}
+	}
+	s.pool.clear()
+	for p in s.bin {
+		if p == 0 {
+			print(ptr_str(p) + ' ouch')
+			continue
+		}
+		unsafe {
+			// println('Freeing from bin')
+			free(p)
+		}
+	}
+	s.bin.clear()
+}