runindex

linear indices for each run in a vector (run length encoding, and decoding)

For a vector V, I = runindex(V) returns a vector I of the same size as
V with linear indices per run within V. Runs are contiguous sequences
of one value for one or more consecutive elements in V.

Example:
% create a vector with four runs: 3x20, 2x4, 4x55, 1x999
V = [20 20 20 4 4 55 55 55 55 999]
I = runindex(V)
% [ 1 2 3 1 2 1 2 3 4 1]

[I, RLE] = rundindex(V) also returns a run length encoding of V in
the N-by-3 array RLE. Each row is a run, with the value in the first
column, the start index in the second column, and the run length in
the third column. N is the number of runs.

V = [8 7 7 7 3 3 12 12 12 12]
[~, RLE] = runindex(V)
% RLE =
% 8 1 1
% 7 2 3
% 3 5 2
% 12 7 4

When given two input vectors U and C, V = runindex(U, C) decodes the
vectors U (values) and C (counts) into the vector V. V is the
concatenation of all values of U, where the k-th value U(k) is
repeated C(k) times. V is a sum(C)-by-1 column vector. When C(k) is 0,
U(k) is omitted from the vector.
[V, RLE] = runindex(U, C) returns the run-length encoding of V in RLE.

V = runindex([5 10 99], [2 1 3])
% V = [5 5 10 99 99 99]

Vin = [1 1 2 2 1 1 1]
[I, RLE] = runindex(Vin) % encoding
% I = [1 2 1 2 1 2 3]
Vout = runindex(RLE(:,1), RLE(:,3)) % decoding
% Vout equals Vin

V = runindex([0 1 0 2], [2 3 1 4])
% V = [0 0 1 1 1 0 2 2 2 2]) ;

See also cumsum, diff, find

Notes
- This function is related to run-length encoding:
https://en.wikipedia.org/wiki/Run-length_encoding