Code echo

Given a nonempty string code and a string stream, both containing only lowercase Latin letters, count the contiguous substrings of stream that are anagrams of code. A match must have the same length and the same number of occurrences of each letter; order may differ. Substrings may overlap, and each starting index counts separately. If stream is shorter than code, return 0.

Example:

count_code_echo("ab", "abab")  →  3

The length-2 substrings are ab, ba and ab. All three contain one a and one b.

count_code_echo("ok", "kooko")  →  3

The length-2 substrings are ko, oo, ok and ko. All except oo contain one o and one k.

Pattern: sliding_window - a frequency window of fixed length

Need a hint?

Reveal one step at a time. Stop when you have enough to keep going.

1 · What to notice

Every candidate has the same length as code. Instead of sorting each substring, what can you update when one letter enters and another leaves?

2 · Walk through an example

For code = ab, stream = ababa, windows are ab, ba, ab, ba. All four have one a and one b, so all count, including overlaps. Keep 26 counts for the target and the current window. After adding a letter, remove the letter that is now outside the fixed window; compare only complete windows.

3 · Pseudocode and self-check

Language-independent pseudocode. Indices start at 0; ← means assignment. Ranges with an upper bound exclude that bound.

size ← LENGTH(code)
expected ← ARRAY(26, 0)
window ← ARRAY(26, 0)
FOR EACH letter IN code:
    expected[LETTER_INDEX(letter)] ← expected[LETTER_INDEX(letter)] + 1
result ← 0
FOR EACH (right, letter) IN stream:
    window[LETTER_INDEX(letter)] ← window[LETTER_INDEX(letter)] + 1
    IF right ≥ size:
        old ← LETTER_INDEX(stream[right - size])
        window[old] ← window[old] - 1
    IF right + 1 ≥ size AND window = expected:
        result ← result + 1
RETURN result

Check your reasoning

code = aa, stream = aaa → 2. A shorter stream → 0. Use counts, not sets: repeated letters matter. Time O(n + m) for the fixed 26-letter alphabet; auxiliary space O(1).

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Code echo

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