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semantic/prototype/Doubt/Matrix.swift
2015-11-12 15:26:50 -05:00

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/// A two-dimensional matrix of memoized values.
///
/// These values are populated by a function from the coordinates of a given cell to the matrixs element type.
///
/// Values are retrieved by subscripting with row/column indices. Out-of-bound indices produce `nil` values, rather than asserting.
public struct Matrix<A, I: ForwardIndexType> {
public init(across: Range<I>, down: Range<I>, compute: (I, I) -> A) {
self.init(across: across, down: down, values: constructRowMajor(across, down: down, forEach: { i, j in Memo { compute(i, j) } }))
}
public let across: Range<I>
public let down: Range<I>
private let width: I.Distance
private let values: [Memo<A>]
public subscript (i: I, j: I) -> Memo<A>? {
guard i != across.endIndex && j != down.endIndex else { return nil }
let i = across.startIndex.distanceTo(i)
let j = down.startIndex.distanceTo(j)
return values[Int((i + j * width).toIntMax())]
}
// MARK: Functor
public func map<Other>(transform: A -> Other) -> Matrix<Other, I> {
return Matrix<Other, I>(across: across, down: down, values: values.map { $0.map(transform) })
}
// MARK: Implementation details
private init(across: Range<I>, down: Range<I>, values: [Memo<A>]) {
self.across = across
self.down = down
self.values = values
self.width = across.count
}
}
/// Constructs a row-major ordering of values produced with `forEach`.
private func constructRowMajor<A, I: ForwardIndexType>(across: Range<I>, down: Range<I>, @noescape forEach: (I, I) -> A) -> [A] {
var values: [A] = []
values.reserveCapacity(Int(across.count.toIntMax()) * Int(down.count.toIntMax()))
for j in down {
for i in across {
values.append(forEach(i, j))
}
}
return values
}
import Memo