'use strict';


const DEFAULT_WEIGHT_MAX = 4;


const graph_proto = {
	connect: function(source, sink, weight) {
		return network_connect(this, source, sink, weight);
	},
	compute: function(inputs, state) {
		return network_compute(this, inputs, state);
	},
};

export function network(input_count, internal_count, output_count, weight_max = 4) {
	const count = input_count + internal_count + output_count;
	const n = Object.create(graph_proto);
	n.input_count = input_count;
	n.output_count = output_count;
	n.adjacency = new Array(count).fill([]);
	n.weight = [];
	return Object.freeze(n);
}


function is_input(n, index) {
	return index < n.input_count;
}
function is_output(n, index) {
	return index >= (n.adjacency.length - n.output_count);
}


function network_connect(n, source, sink, weight) {
	if (is_input(n, sink)) {
		// inputs cannot be sinks
		throw new Error("attempt to use input as sink");
	}
	if (is_output(n, source)) {
		// outputs cannot be sources
		throw new Error("attempt to use output as source");
	}

	const nn = Object.create(graph_proto);
	nn.input_count = n.input_count;
	nn.output_count = n.output_count;
	nn.adjacency = n.adjacency.map((row, i) => {
		if (i === source && i === sink) {
			// self-loop
			return [...row, 2];
		} else if (i === source) {
			return [...row, 1];
		} else if (i === sink) {
			return [...row, -1];
		} else {
			return [...row, 0];
		}
	});
	nn.weight = [...n.weight, weight];

	return Object.freeze(nn);
}


function incident_edges(n, adj) {
	const incident = adj
		.map((edge, index) => edge < 0 ? index : null)
		.filter(index => index !== null);

	return incident;
}


// get the indices of the ends of an edge
// in the case of self-loops, both values are the same
function edge_ends(n, edge) {
	const ends = n.adjacency
		.map((adj, index) => adj[edge] !== 0 ? index : null)
		.filter(index => index != null);
	
	ends.sort((a, b) => n.adjacency[a][edge] < n.adjacency[b][edge] ? -1 : 1);

	if (ends.length === 1) {
		return { source: ends[0], sink: ends[0] };
	} else if (ends.length === 2) {
		return { source: ends[1], sink: ends[0] };
	} else {
		throw new Error("something bad happened with the ends");
	}
}


function get_value(n, index, input, cache) {
	if (cache !== undefined && cache[index]) {
		return cache[index];
	}
	if (is_input(n, index)) {
		return input[index];
	}
	const adj = n.adjacency[index];
	const incident = incident_edges(n, adj);	
	const weight = incident.map(x => n.weight[x]);
	const sources = incident
		.map(x => edge_ends(n, x).source);

	const sum = sources
		.reduce((acc, x, i) => 
			acc + (weight[i] * get_value(n, x, input, cache)), 
			0
		);

	const value = Math.tanh(sum);

	// !!! impure caching !!!
	if (cache !== undefined) {
		cache[index] = value;
	}
	
	return value;
}


function network_compute(n, input, state) {
	// !!! impure caching !!!
	const value_cache = {};

	const hidden = n.adjacency
		.map((x, i) => 
			(
				(!(is_input(n, i))) && 
				(!is_output(n, i))) ? i : null)
		.filter(i => i !== null);
	
	const outputs = n.adjacency
		.map((x, i) => is_output(n, i) ? i : null)
		.filter(i => i !== null);

	const output = Object.freeze(
		outputs.map(x => get_value(n, x, input, value_cache))
	);

	const newstate = Object.freeze(
		hidden.map(x => get_value(n, x, input, value_cache))
	);
	
	return Object.freeze([output, newstate]);
}