Partition a Target String by Source Substrings

Problem Statement

You are given two strings, source and target. Determine the minimum number of contiguous substrings of source that can be concatenated together to form target.

If it is impossible to construct target using only contiguous substrings of source, return -1.

Function Signature

def min_source_substrings(source: str, target: str) -> int:
    pass

Input & Output Format

• Input: Two non-empty strings, source and target, consisting of lowercase English letters.
• Output: An integer representing the minimum number of segments, or -1 if impossible.

Constraints

• 1 <= len(source) <= 5 * 10^4
• 1 <= len(target) <= 5 * 10^4
• source and target consist of lowercase English letters ('a'–'z').

Examples

Example 1

Input: source = "abcde", target = "abcab"
Output: 2
Explanation:
We can partition target into "abc" and "ab".
Both "abc" and "ab" are contiguous substrings of source.
Hence, 2 substrings are required.

Example 2

Input: source = "banana", target = "naban"
Output: 2
Explanation:
We can partition target into "na" and "ban", both of which appear as substrings in "banana".

Example 3

Input: source = "abc", target = "abcz"
Output: -1
Explanation:
The character 'z' does not appear in source, making it impossible to form target.

Edge Cases to Consider

• target is identical to or already a substring of source (output should be 1).
• A character in target does not exist in source (immediate return -1).
• Every character must be taken individually (e.g., reversed strings with no multi-character overlap).