Add string utilities for splitting strings

/* A collection of string utilities for OpenSCAD.

These functions provide basic string manipulation features that are

not available natively in stable OpenSCAD.

*/

// SPDX-License-Identifier: CERN-OHL-S-2.0

// For copyright and authorship information, see the Git history at:

// https://gitlab.com/openflexure/openflexure-microscope

/* Concatenate a list of strings into a single string.

This function reduces the list recursively, concatenating

pairs of elements until a single string remains.

:param input: A non-empty list of strings

:return: A single string formed by concatenating all elements of `input`

:assert len(input) > 0: Checks the list isn't empty

*/

function str_cat(input) =

    assert(len(input) > 0, "str_cat cannot handle empty lists")

    len(input) == 1 ? input[0] :

       len(input) == 2 ? str(input[0], input[1]) :

           str_cat(

               concat(

                   [str(input[0], input[1])],

                   [for (i = [2:len(input)-1]) input[i]]

               )

           );

/* Return a substring using inclusive start and end indices.

No bounds checking is performed. Supplying indices outside

the valid range may result in undefined behaviour.

:param string: The source string

:param start: Starting index (inclusive, zero-based)

:param end: Ending index (inclusive, zero-based)

:return: The substring from `start` to `end`

*/

function sub_str(string, start, end) =

    str_cat([ for (i = [start:end]) string[i] ]);

/* Split a milti-line string into a list of one-line strings.

:param string: The input string

:return: A list of strings, one per line

*/

function split_string_lines(string) = let(

    // get position of any \n

    pos = search("\n", string, 0)[0],

    s_pos = concat([0], [for (p=pos) p+1]),

    e_pos = concat( [for (p=pos) p-1], len(string)-1)

) [for (i = [0:len(pos)]) sub_str(string, s_pos[i], e_pos[i])];