static inline void ppu_tick(struct nes_state *state);
static inline uint8_t memory_read(struct nes_state *state, uint32_t offset);

// DMC frequency table (NTSC)
static const uint16_t dmc_rate_table[16] = {
	428, 380, 340, 320, 286, 254, 226, 214,
	190, 160, 142, 128, 106,  85,  72,  54
};

// $4015 write
static void apu_write4015(struct nes_state *state, uint8_t val) {
	struct apu_state *apu = &state->apu;

	if(val & 0x10) {
		if(!apu->dmc_dma_enabled || apu->dmc_bytes_remaining == 0) {
			apu->dmc_current_addr = 0xC000 + ((uint16_t)apu->dmc_sample_addr << 6);
			apu->dmc_bytes_remaining = ((uint16_t)apu->dmc_sample_len << 4);
		}
		apu->dmc_dma_enabled = 1;
	} else {
		apu->dmc_dma_enabled = 0;
	}
}

// $4015 read
static uint8_t apu_read4015(struct nes_state *state) {
	struct apu_state *apu = &state->apu;
	uint8_t result = 0;

	if(apu->dmc_bytes_remaining > 0) {
		result |= 0x10;
	}
	if(apu->irq_pending) {
		result |= 0x40;
		apu->irq_pending = 0;
	}
	return result;
}

// $4010–$4013, $4015 write
static void apu_write(struct nes_state *state, uint16_t addr, uint8_t val) {
	struct apu_state *apu = &state->apu;

	switch(addr) {
		case 0x4010: {
			apu->dmc_irq_enable = (val >> 7) & 1;
			apu->dmc_loop_flag = (val >> 6) & 1;
			apu->dmc_freq_index = val & 0x0F;
		} break;
		case 0x4011: {
			// DAC write ignored
		} break;
		case 0x4012: {
			apu->dmc_sample_addr = val;
		} break;
		case 0x4013: {
			apu->dmc_sample_len = val;
		} break;
		case 0x4015: {
			apu_write4015(state, val);
		} break;
	}
}

// APU tick
static inline void apu_tick(struct nes_state *state) {
	struct apu_state *apu = &state->apu;

	apu->frame_cycle++;

	if(apu->mode == 0) {
		if(apu->frame_cycle == 7457) {
			// Quarter frame
		} else if(apu->frame_cycle == 14913) {
			// Quarter frame
			// Half frame
		} else if(apu->frame_cycle == 22371) {
			// Quarter frame
		} else if(apu->frame_cycle == 29829) {
			// Half frame
			if(!apu->irq_inhibit) {
				apu->irq_pending = 1;
			}
		} else if(apu->frame_cycle >= 29830) {
			apu->frame_cycle = 0;
		}
	} else {
		if(apu->frame_cycle == 7457) {
			// Quarter frame
		} else if(apu->frame_cycle == 14913) {
			// Quarter frame
			// Half frame
		} else if(apu->frame_cycle == 22371) {
			// Quarter frame
		} else if(apu->frame_cycle == 29829) {
			// Half frame
		} else if(apu->frame_cycle == 37281) {
			// Quarter frame
		} else if(apu->frame_cycle >= 37282) {
			apu->frame_cycle = 0;
		}
	}

	// if(apu->mode == 0) {
	// 	if(apu->frame_cycle == 7457 || apu->frame_cycle == 14913 || apu->frame_cycle == 22371) {
	// 		// Quarter frame
	// 	}
	// 	if(apu->frame_cycle == 14913 || apu->frame_cycle == 29829) {
	// 		// Half frame
	// 	}
	// 	if(apu->frame_cycle == 29829 && !apu->irq_inhibit) {
	// 		apu->irq_pending = 1;
	// 	}
	// 	if(apu->frame_cycle >= 29830) {
	// 		apu->frame_cycle = 0;
	// 	}
	// } else {
	// 	if(apu->frame_cycle == 7457 || apu->frame_cycle == 14913 || apu->frame_cycle == 22371 || apu->frame_cycle == 37281) {
	// 		// Quarter frame
	// 	}
	// 	if(apu->frame_cycle == 14913 || apu->frame_cycle == 29829) {
	// 		// Half frame
	// 	}
	// 	if(apu->frame_cycle >= 37282) {
	// 		apu->frame_cycle = 0;
	// 	}
	// }

	if(apu->dmc_dma_enabled && apu->dmc_bytes_remaining > 0) {
		apu->dmc_sample_timer++;
		if(apu->dmc_sample_timer >= dmc_rate_table[apu->dmc_freq_index]) {
			apu->dmc_sample_timer = 0;

			uint8_t val = memory_read(state, apu->dmc_current_addr);
			(void)val;

			apu->dmc_current_addr++;
			if(apu->dmc_current_addr == 0x0000) {
				apu->dmc_current_addr = 0x8000;
			}

			apu->dmc_bytes_remaining--;

			if(apu->dmc_bytes_remaining == 0) {
				if(apu->dmc_loop_flag) {
					apu->dmc_current_addr = 0xC000 + ((uint16_t)apu->dmc_sample_addr << 6);
					apu->dmc_bytes_remaining = ((uint16_t)apu->dmc_sample_len << 4);
				} else {
					if(apu->dmc_irq_enable) {
						apu->irq_pending = 1;
					}
					apu->dmc_dma_enabled = 0;
				}
			}

			for(uint32_t i = 0; i < 4; i++) {
				state->cpu.cycles++;
				ppu_tick(state);
			}
		}
	}
}