[Issue 6788] std.range.pairwise?

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=6788



--- Comment #4 from bearophile_hugs at eml.cc 2013-03-12 12:04:54 PDT ---
Basic version for a random access range:


import std.range: ForeachType, isRandomAccessRange, hasLength;
import std.traits: Unqual, isNarrowString;
import std.typecons: Tuple;

struct Pairwise(Range) {
    alias R = Unqual!Range;
    alias Pair = Tuple!(ForeachType!R, "a", ForeachType!R, "b");
    R _input;
    size_t i, j;

    this(Range r_) {
        this._input = r_;
        j = 1;
    }

    @property bool empty() {
        return j >= _input.length;
    }

    @property Pair front() {
        return Pair(_input[i], _input[j]);
    }

    void popFront() {
        if (j >= _input.length - 1) {
            i++;
            j = i + 1;
        } else {
            j++;
        }
    }
}

Pairwise!Range pairwise(Range)(Range r)
if (isRandomAccessRange!Range && hasLength!Range && !isNarrowString!Range) {
    return typeof(return)(r);
}

import std.stdio: writeln;

void main() {
    (new int[0]).pairwise.writeln;
    [10].pairwise.writeln;
    [10, 20].pairwise.writeln;
    [10, 20, 30].pairwise.writeln;
    [10, 20, 30, 40].pairwise.writeln;
}



Once combinations() is implemented that code can be replaced with something
like:

auto pairwise(Range)(Range r) if (...) {
    alias R = Unqual!Range;
    alias Pair = Tuple!(ForeachType!R, "a", ForeachType!R, "b");    
    return combinations(r, 2).map(t => Pair(t.tupleof));
}

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