Date: 2026-06-06
Time: 18:39
This file implements LeetCode #157 — Read N Characters Given Read4. It solves the problem of reading exactly n characters from a file using only a low-level read4 API that reads at most 4 characters at a time. The file is self-contained: it defines the simulated API (Reader4), the solution (Solution), and a full test suite (TestSolution).
Reader4 (base class, lines 7–22)Simulates the read4 API that LeetCode provides but doesn't let you see. It wraps a string as a virtual file with an internal cursor (_pos).
_init_(self, file: str) — stores the file content and initializes position to 0.read4(self, buf4: List[str]) -> int — reads up to 4 characters into buf4, advances _pos, returns the actual count read. This is the only I/O primitive the solution is allowed to use.Solution (lines 25–46, extends Reader4)read(self, buf: List[str], n: int) -> int — the core algorithm. Repeatedly calls read4 into a temporary 4-element buffer, copies the lesser of count (chars actually read) and n - total (chars still needed) into buf, and stops when either n characters have been collected or read4 returns fewer than 4 (EOF reached).TestSolution (lines 49–89)Nine test cases exercising boundary conditions: file shorter than n, exact match, file longer than n, single-char files, reads aligned and unaligned to 4-byte boundaries, and n much larger than the file.
Adapter/Inheritance pattern: Solution extends Reader4 to gain access to read4, mirroring LeetCode's problem structure where the solution class inherits the API. This is the standard LeetCode convention for "given an API" problems.
Buffered reading: The 4-element temporary buffer (buf4) acts as a small intermediary between the fixed-size read primitive and the variable-size output buffer — the classic pattern for bridging mismatched I/O granularities.
Copy-what-you-need with min: to_copy = min(count, n - total) prevents over-reading past the requested n, which is the key correctness detail.
Imports: unittest (stdlib) and List from typing. No external dependencies.
Imported by: The "Imported By" list in the prompt is misleading — those are unrelated test files across the repo that happen to import unittest, not actual consumers of this module. The only real dependent is read-n-characters-given-read4/test_solution.py, and the tests are already inline in this file.
1. Caller creates Solution("some file content"), which initializes Reader4._pos = 0.
2. Caller allocates a destination buffer buf of size >= n and calls sol.read(buf, n).
3. read loops:
read4(buf4) → fills up to 4 chars, returns count.to_copy = min(count, n - total).tocopy chars from buf4 into buf[total..total+tocopy].total.count < 4, the file is exhausted → break.4. Returns total, the number of characters actually placed in buf.
total <= n always holds — enforced by the while total < n guard and the min(count, n - total) cap on each copy.read4 returns 0–4 — guaranteed by the while count < 4 loop and the file-length bound.read is designed to be called once per Solution instance (no internal buffer carryover). This distinguishes it from LeetCode #158 (Read N Characters Given Read4 II — Call Multiple Times).buf must be pre-allocated to at least n elements by the caller.None. The code assumes valid inputs — buf is large enough, n >= 0, and file is a string. This is typical for LeetCode solutions where input constraints are guaranteed by the problem.
read4-ii-multiple-calls — The follow-up problem (LeetCode #158) where leftover bytes in buf4 must be carried across calls, requiring an instance-level buffer — a significant design change from this single-call version.read-n-characters-given-read4/solution.py:read — Trace what happens when n is not a multiple of 4 (e.g., n=7 on a 10-char file) to understand why min(count, n - total) is load-bearing.read-n-characters-given-read4/review.md — Check whether the review captures the single-call vs. multi-call distinction and any noted edge cases.buffered-io-pattern — This pattern (small fixed-size reads assembled into variable-size output) recurs in systems programming — compare with how fread/BufferedReader work.read4-reads-at-most-4 — Reader4.read4 always returns a value in [0, 4] and advances the internal position by exactly that amount.solution-read-never-exceeds-n — Solution.read places at most n characters into buf, enforced by min(count, n - total) on every copy iteration.solution-is-single-call — Solution.read has no instance-level carryover buffer, so calling it a second time on the same instance resumes from wherever _pos stopped, but any leftover bytes from the last read4 call are lost.eof-detected-by-short-read — The loop terminates early when read4 returns fewer than 4, which is the sole EOF signal.