import math
from collections import deque

from . import _dataset


def look_downwards(data, x, start):
	for i in range(start, 0, -1):
		if data[i - 1].x < x:
			break
	else:
		raise IndexError
	return i - 1


def look_upwards(data, x, start):
	for i in range(start, len(data)):
		if data[i + 1].x > x:
			break
	else:
		raise IndexError
	return i


def find_segment(data, x):
	width = data[-1].x - data[0].x
	relative = x - data[0].x
	candidate = math.floor(relative / width * len(data))
	look = look_downwards if data[candidate].x > x else look_upwards  # May raise IndexError
	candidate = look(data, x, candidate)
	return candidate, candidate + 1


def find_boundary_curves(swis, x, y):
	segments = deque()
	for index, data in swis:
		i, j = find_segment(data, x)
		if data[i].y > y and data[j].y > y:
			segments.append((index, data, i, j))
			break
		if data[i].y < y and data[j].y < y:
			if segments:
				segments.popleft()
		segments.append((index, data, i, j))
	if len(segments) == 3:
		middle = segments[1][1]
		run = middle[j].x - middle[i].x
		if run == 0:
			raise RuntimeError  # tidy up dataset
		slope = (middle[j].y - middle[i].y) / run
		intercept = middle[j].y - slope * middle[j].x
		value = slope * x + intercept
		if value == y:
			raise RuntimeError  # Exactly on point; SWI == index
		if value < y:
			segments.popleft()
		else:
			segments.pop()
	if len(segments) == 1:
		raise RuntimeError  # SWI == -10
	return segments


def calculate_swi(x, y):
	low, high = find_boundary_curves(_dataset.INDICES, x, y)
	vec = _dataset.Vector(x, y)
	dist_to_low = min(abs(vec - p) for p in (low[1][low[2]], low[1][low[2]]))
	dist_to_high = min(abs(vec - p) for p in (high[1][high[2]], high[1][high[2]]))
	return dist_to_low / (dist_to_low + dist_to_high) * (high[0] - low[0]) + low[0]