On Tue, 24 May 2022 17:53:38 GMT, Maurizio Cimadamore <mcimadam...@openjdk.org> wrote:
>> Indices start at zero. The ceilDiv operation is needed so that the operation >> returns the first index outisde the range (it's a bit subtle, sorry, but I >> don't know how else to express). > > Here's a concrete example: > > Consider a sequence layout with 6 elements. Then: > > element count = 6 > valid indices 0, 1, 2, 3, 4, 5 > > Now consider a var handle that is obtained by calling the path element > method, passing the following parameters > > start = 1 > step = 2 > > This sets up the following mapping between logical an physical indices: > > 0 -> 1 > 1 -> 3 > 2 -> 5 > > Where on the LHS we have the logical index (the one passed to the VH) and on > the RHS we have the actual index it is translated to. > > Note that the index map has three elements. So the upper bound (exclusive) of > the index map is 3 - that is, we can pass indices 0, 1, 2. > > According to the formula shown in the javadoc: > > B = ceilDiv((elementCount - start) / step); > > so: > > B = ceilDiv((6 - 1) / 2) > = ceilDiv(5 / 2) > > Note how, w/o ceilDiv we'd get 2 (the last valid index), and not 3 (the first > invalid index). The terms `x_1, x_2, ... x__n` are defined, but `x_0` is not. I think you can refer to the first index out of bounds as the exclusive upper bound of the range? ------------- PR: https://git.openjdk.java.net/jdk/pull/8868
