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var AbstractSemantics = require("./abstract-semantics").AbstractSemantics;

var empty = {}; // used for getting properties of falsy objects

var Semantics = exports.Semantics = AbstractSemantics.create(AbstractSemantics, {

    operators: {
        value: {

            // from highest to lowest precedence

            // unary
            not: function (a) {
                return !a;
            },

            // function calls
            startsWith: function (a, b) {
                return a.length >= b.length &&
                    this.equals(a.slice(0, b.length), b);
            },
            endsWith: function (a, b) {
                return a.length >= b.length &&
                    this.equals(a.slice(a.length - b.length, a.length), b);
            },

            // algebra

            // exponential
            pow: function (a, b) {
                return Math.pow(a, b);
            },

            // multiplicative
            mul: function (a, b) {
                return a * b;
            },
            div: function (a, b) {
                return a / b;
            },
            mod: function (a, b) {
                return a % b;
            },

            // arithmetic
            add: function (a, b) {
                return a + b;
            },
            sub: function (a, b) {
                return a - b;
            },

            // transitive relations
            lessThan: function (a, b) {
                return Semantics.compare(a, b) < 0;
            },
            lessThanOrEquals: function (a, b) {
                return Semantics.compare(a, b) <= 0;
            },
            greaterThan: function (a, b) {
                return Semantics.compare(a, b) > 0;
            },
            greaterThanOrEquals: function (a, b) {
                return Semantics.compare(a, b) >= 0;
            },

            // equivalence relations
            equals: function (a, b) {
                if (a == null || b == null) {
                    return a === b;
                } else if (a.equals) {
                    return a.equals(b);
                } else if (b.equals) {
                    return b.equals(a);
                } else {
                    return a === b;
                }
            },
            notEquals: function (a, b) {
                return !this.equals(a, b);
            },

            // logical ('and' and 'or' are evaluators for the short-circuiting)

            xor: function (a, b) {
                return a !== b;
            },

            // collection operators

            contains: function (collection, value) { // pertains to strings
                return collection.indexOf(value) !== -1;
            },
            has: function (collection, value) { // pertains to other collections
                return collection.indexOf(value) !== -1;
            },

            slice: function (collection, start, stop) {
                return collection.slice(start, stop);
            },
            // TODO rewrite to accept length then from value
            sliceFrom: function (collection, value, length) {
                var start = collection.indexOf(value);
                if (start !== -1) {
                    length = Math.min(collection.length - start, length);
                    return collection.slice(start, start + length);
                } else {
                    return [];
                }
            }

        }
    },

    evaluators: {
        value: {

            // properties
            get: function (evaluateObject, evaluateKey, parents) {

                // construct remaining path
                var remainingPath = [];
                parents = parents.parents;
                while (parents) {
                    if (parents.syntax.type !== 'get')
                        break;
                    remainingPath.push(parents.syntax.args[1].value);
                    parents = parents.parents;
                }
                if (remainingPath.length === 0) {
                    remainingPath = null;
                } else {
                    remainingPath = remainingPath.join(".");
                }

                return function (value, parameters, visitor) {
                    var object = evaluateObject(value, parameters, visitor);
                    var key = evaluateKey(value, parameters, visitor);
                    var result = (object || empty)[key];
                    if (visitor) {
                        visitor(object, key, result, remainingPath);
                    }
                    return result;
                };
            },

            // logical
            and: function (a, b) {
                return function (value, parameters) {
                    return a(value, parameters) && b(value, parameters);
                };
            },
            or: function (a, b) {
                return function (value, parameters) {
                    return a(value, parameters) || b(value, parameters);
                };
            },

            // ternary conditional
            'if': function (guard, consequent, alternate) {
                return function (value, parameters) {
                    var flag = guard(value, parameters);
                    if (flag === true) {
                        return consequent(value, parameters);
                    } else if (flag === false) {
                        return alternate(value, parameters);
                    } else {
                        throw new Error("Expected true or false, got: " + JSON.stringify(flag));
                    }
                };
            },

            it: function (a, b) {
                return function (value, parameters) {
                    return b(a(value, parameters), parameters);
                }
            },

            one: function (a, b) {
                return function (value, parameters) {
                    var object = a(value, parameters)[0]; // handle other collections
                    return b(object, parameters);
                };
            },
            only: function (a, b) {
                return function (value, parameters) {
                    var objects = a(value, parameters); // handle other collections
                    if (objects.length !== 1)
                        return false;
                    return b(objects[0], parameters);
                };
            },
            filter: function (a, b) {
                return function (value, parameters) {
                    return a(value, parameters)
                    .filter(function (object) {
                        return b(object, parameters);
                    });
                };
            },
            every: function (a, b) {
                return function (value, parameters) {
                    return a(value, parameters)
                    .every(function (object) {
                        return b(object, parameters);
                    });
                };
            },
            some: function (a, b) {
                return function (value, parameters) {
                    return a(value, parameters)
                    .some(function (object) {
                        return b(object, parameters);
                    });
                };
            },
            map: function (a, b) {
                return function (value, parameters) {
                    return a(value, parameters)
                    .map(function (object) {
                        return b(object, parameters);
                    });
                };
            },

            sorted: function (collection, by, descending) {
                var semantics = this;
                return function (value, parameters) {
                    var order = descending() ? -1 : 1;
                    return collection(value, parameters)
                    .map(function (item) {
                        return {
                            by: by(item, parameters),
                            value: item
                        };
                    })
                    .sort(function (a, b) {
                        return semantics.compare(a.by, b.by) * order;
                    })
                    .map(function (pair) {
                        return pair.value;
                    })
                }
            },

            // reductions
            sum: function (collection, modify) {
                var self = this;
                return function (value, parameters) {
                    var value = self.sum(collection(value, parameters));
                    return modify(value, parameters);
                };
            },
            count: function (collection, modify) {
                var self = this;
                return function (value, parameters) {
                    var value = self.count(collection(value, parameters));
                    return modify(value, parameters);
                };
            },
            average: function (collection, modify) {
                var self = this;
                return function (value, parameters) {
                    var value = self.average(collection(value, parameters));
                    return modify(value, parameters);
                };
            },
            unique: function (collection, modify) {
                var self = this;
                return function (value, parameters) {
                    var value = self.unique(collection(value, parameters));
                    return modify(value, parameters);
                };
            },
            flatten: function (collection, modify) {
                var self = this;
                return function (value, parameters) {
                    var value = self.flatten(collection(value, parameters));
                    return modify(value, parameters);
                };
            }

        }
    },

    compile: {
        value: function (syntax, parents) {
            var self = this;
            var child = {
                syntax: syntax,
                parents: parents
            };
            if (syntax.type === 'array') {
                var terms = syntax.terms.map(function (term) {
                    return self.compile(term, child);
                });
                return function (value, parameters) {
                    return terms.map(function (term) {
                        return term(value, parameters);
                    });
                };
            } else {
                return AbstractSemantics.compile.call(self, syntax, parents);
            }
        }
    },

    compare: {
        value: function (a, b) {
            if (typeof a !== typeof b)
                return 0;
            if (a === b)
                return 0;
            if (typeof a === "number")
                return a - b;
            if (typeof a === "string")
                return a < b ? -1 : 1;
            if (Array.isArray(a)) {
                if (!Array.isArray(b))
                    return 0;
                var length = Math.min(a.length, b.length);
                for (var i = 0; i < length; i++) {
                    var comparison = this.compare(a[i], b[i]);
                    if (comparison)
                        return comparison;
                }
                return a.length - b.length;
            }
            if (typeof a.compare === "function")
                return a.compare(b);
            if (typeof b.compare === "function")
                return -b.compare(a);
            if (typeof a.lessThan === "function" && typeof a.equals === "function")
                return a.equals(b) ? 0 : a.lessThan(b) ? -1 : 1;
            return 0;
        }
    },

    count: {
        value: function (collection) {
            if (typeof collection.count === "function") {
                return collection.count();
            } else {
                return collection.length;
            }
        }
    },

    sum: {
        value: function (collection) {
            return collection.reduce(function (a, b) {
                return a + b;
            }, 0);
        }
    },

    average: {
        value: function (collection) {
            return this.sum(collection) / this.count(collection);
        }
    },

    unique: {
        value: function (collection) {
            var unique = [];
            collection.forEach(function (value) {
                if (unique.every(function (uniqueValue) {
                    return Semantics.compare(value, uniqueValue) !== 0;
                })) {
                    unique.push(value);
                }
            });
            return unique;
        }
    },

    flatten: {
        value: function (table) {
            return table.reduce(function (flat, row) {
                return flat.concat(row);
            }, [])
        }
    }

});