Description of Change

Observe that in the old code, because the <= is used in the loop, there is an index out of bounds error. The main reason for this PR is to fix this problem. The rest is just fix as you go:

• std::size_t should be used for indexing standard containers,
• the api is slightly more intuitive now (consider calling the build function twice in the old code).

Checklist

• Added description of change
• PR title follows semantic commit guidelines
• Search previous suggestions before making a new one, as yours may be a duplicate.
• I acknowledge that all my contributions will be made under the project's license.

Notes:
Fix one-off error leading to index out of bounds.