Git দিয়ে bug খোঁজা — bisect, blame আর log -S

$ git log --oneline --decorate
2cd69ab (HEAD -> main) Add a thank-you line to the receipt
b8fcb04 Describe version 1.1 in README
22367c8 Update flour price
05792ca Move format_taka next to line_total
3c65833 Add a test for line_total
1853d66 Move eid discount into discount.py
87f8469 Format code: spaces around operators, double quotes
7331e00 Add delivery charge
70c0892 Use a constant for the discount threshold (#17)
c71b112 Add sugar and flour prices
3765778 Finish receipt printing
a2aba58 WIP: start receipt printing
392acc2 Add usage notes to README
3198235 Update onion price
a122fb6 (tag: v1.0) Add format_taka helper
7bd5ea2 Apply eid discount to the total
86bd46b Add eid discount function
0a0b885 Round the total to two decimals
40f5f49 Add bill calculator
90e700e Add price list and README

import sys
sys.path.insert(0, ".")
try:
    import bill
except SyntaxError:
    print("cannot import bill.py - skip")
    sys.exit(125)
bill.load_prices("prices.txt")
got = bill.final_total([("oil", 5)])     # 5 x 200 = 1000 taka
print("total for 1000 taka:", got)
sys.exit(0 if got == 950.0 else 1)

$ python ../check.py
total for 1000 taka: 1000.0
$ echo $?
1
$ git switch --detach v1.0
HEAD is now at a122fb6 Add format_taka helper
$ python ../check.py
total for 1000 taka: 950.0
$ git switch main
Previous HEAD position was a122fb6 Add format_taka helper
Switched to branch 'main'

// Bisect as binary search: commit 0 is known good, commit 999 is known bad.
function mulberry32(a) { return function () { a |= 0; a = (a + 0x6D2B79F5) | 0;
  let t = Math.imul(a ^ (a >>> 15), 1 | a); t = (t + Math.imul(t ^ (t >>> 7), 61 | t)) ^ t;
  return ((t ^ (t >>> 14)) >>> 0) / 4294967296; }; }
const N = 1000;
function bisect(seed) {
  const rand = mulberry32(seed);
  const firstBad = 1 + Math.floor(rand() * (N - 1));    // commits >= firstBad are bad
  const broken = new Set();                             // about 5% cannot be built
  for (let c = 1; c < N - 1; c++) if (rand() < 0.05) broken.add(c);
  let good = 0, bad = N - 1, runs = 0, skips = 0; const skipped = new Set();
  while (bad - good > 1) {
    const mid = Math.floor((good + bad) / 2);
    let pick = -1;                                      // nearest commit not yet skipped
    for (let d = 0; pick < 0 && d < bad - good; d++)
      for (const c of [mid + d, mid - d])
        if (pick < 0 && c > good && c < bad && !skipped.has(c)) pick = c;
    if (pick < 0) return { seed, firstBad, result: "only skipped commits left", runs, skips };
    runs++;
    if (broken.has(pick)) { skipped.add(pick); skips++; continue; }   // exit 125
    if (pick >= firstBad) bad = pick; else good = pick;
  }
  return { seed, firstBad, result: bad === firstBad ? "found " + bad : "WRONG " + bad, runs, skips };
}
console.log("candidates =", N - 1, "| ceil(log2) =", Math.ceil(Math.log2(N - 1)));
for (const seed of [55, 24, 10]) {
  const r = bisect(seed);
  console.log(`seed ${String(seed).padEnd(3)} first bad ${r.firstBad} -> ${r.result}, runs ${r.runs}, skips ${r.skips}`);
}
let found = 0, unsure = 0, runs = 0, maxVerdicts = 0;
for (let seed = 1; seed <= 1000; seed++) {
  const r = bisect(seed); if (r.result.startsWith("found")) found++; else unsure++;
  runs += r.runs; maxVerdicts = Math.max(maxVerdicts, r.runs - r.skips);
}
console.log(`1000 sessions: found ${found}, only-skipped ${unsure}, mean runs ${(runs / 1000).toFixed(2)}, max verdicts ${maxVerdicts}`);

$ git bisect start
status: waiting for both good and bad commits
$ git bisect bad
status: waiting for good commit(s), bad commit known
$ git bisect good v1.0
Bisecting: 6 revisions left to test after this (roughly 3 steps)
[7331e0026c8ab498791f4f3f8c62dfd007b1da66] Add delivery charge
$ python ../check.py
total for 1000 taka: 1000.0
$ git bisect bad
Bisecting: 3 revisions left to test after this (roughly 2 steps)
[a2aba5872252155ccead4e4e1a8e6301a597184d] WIP: start receipt printing
$ python ../check.py
cannot import bill.py - skip
$ git bisect skip
Bisecting: 2 revisions left to test after this (roughly 2 steps)
[376577801e539bcc8b96af38db80a06871369509] Finish receipt printing
$ python ../check.py
total for 1000 taka: 950.0
$ git bisect good
Bisecting: 0 revisions left to test after this (roughly 1 step)
[70c0892dbff8ce3bd88d0b4c0a628b00a7a9d5e6] Use a constant for the discount threshold (#17)
$ python ../check.py
total for 1000 taka: 1000.0
$ git bisect bad
Bisecting: 0 revisions left to test after this (roughly 0 steps)
[c71b11297237a8405fd2077019e1cdef10312046] Add sugar and flour prices
$ python ../check.py
total for 1000 taka: 950.0
$ git bisect good
70c0892dbff8ce3bd88d0b4c0a628b00a7a9d5e6 is the first bad commit
commit 70c0892dbff8ce3bd88d0b4c0a628b00a7a9d5e6
Author: Rahim <rahim@example.com>
Date:   Thu Jan 15 10:11:00 2026 +0600

    Use a constant for the discount threshold (#17)

    Issue #17: keep the eid discount threshold in one place
    so that it can be changed before each Eid.

 bill.py | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

$ git show 70c0892 -- bill.py
…
@@ -16,8 +16,10 @@ def bill_total(cart):
         total += line_total(item, qty)
     return round(total, 2)

+DISCOUNT_MIN = 1000
+
 def eid_discount(amount):
-    if amount >= 1000:
+    if amount > DISCOUNT_MIN:
         return amount*0.95
     return amount
 

$ git bisect log
git bisect start
# status: waiting for both good and bad commits
# bad: [2cd69ab26193187ec54dd3d945bf624c8ef132b7] Add a thank-you line to the receipt
git bisect bad 2cd69ab26193187ec54dd3d945bf624c8ef132b7
# status: waiting for good commit(s), bad commit known
# good: [a122fb65eb5929fb827238c12eb660f083eb7e4f] Add format_taka helper
git bisect good a122fb65eb5929fb827238c12eb660f083eb7e4f
# bad: [7331e0026c8ab498791f4f3f8c62dfd007b1da66] Add delivery charge
git bisect bad 7331e0026c8ab498791f4f3f8c62dfd007b1da66
# skip: [a2aba5872252155ccead4e4e1a8e6301a597184d] WIP: start receipt printing
git bisect skip a2aba5872252155ccead4e4e1a8e6301a597184d
# good: [376577801e539bcc8b96af38db80a06871369509] Finish receipt printing
git bisect good 376577801e539bcc8b96af38db80a06871369509
# bad: [70c0892dbff8ce3bd88d0b4c0a628b00a7a9d5e6] Use a constant for the discount threshold (#17)
git bisect bad 70c0892dbff8ce3bd88d0b4c0a628b00a7a9d5e6
…
$ git bisect log > ../bisect.log
$ git bisect reset
Previous HEAD position was c71b112 Add sugar and flour prices
Switched to branch 'main'

$ git bisect visualize --oneline
70c0892 Use a constant for the discount threshold (#17)
c71b112 Add sugar and flour prices

$ git bisect start --term-old=fast --term-new=slow
status: waiting for both good and bad commits
$ git bisect slow HEAD
status: waiting for good commit(s), bad commit known
$ git bisect fast v1.0
Bisecting: 6 revisions left to test after this (roughly 3 steps)
[7331e0026c8ab498791f4f3f8c62dfd007b1da66] Add delivery charge
$ git bisect terms
Your current terms are fast for the old state
and slow for the new state.
$ git bisect good
error: Invalid command: you're currently in a slow/fast bisect
fatal: unknown command: 'good'
…

$ git bisect start HEAD v1.0
Bisecting: 6 revisions left to test after this (roughly 3 steps)
[7331e0026c8ab498791f4f3f8c62dfd007b1da66] Add delivery charge
$ git bisect run python ../check.py
running 'python' '../check.py'
total for 1000 taka: 1000.0
Bisecting: 3 revisions left to test after this (roughly 2 steps)
[a2aba5872252155ccead4e4e1a8e6301a597184d] WIP: start receipt printing
running 'python' '../check.py'
cannot import bill.py - skip
Bisecting: 2 revisions left to test after this (roughly 2 steps)
[376577801e539bcc8b96af38db80a06871369509] Finish receipt printing
running 'python' '../check.py'
total for 1000 taka: 950.0
Bisecting: 0 revisions left to test after this (roughly 1 step)
[70c0892dbff8ce3bd88d0b4c0a628b00a7a9d5e6] Use a constant for the discount threshold (#17)
running 'python' '../check.py'
total for 1000 taka: 1000.0
Bisecting: 0 revisions left to test after this (roughly 0 steps)
[c71b11297237a8405fd2077019e1cdef10312046] Add sugar and flour prices
running 'python' '../check.py'
total for 1000 taka: 950.0
70c0892dbff8ce3bd88d0b4c0a628b00a7a9d5e6 is the first bad commit
…
bisect found first bad commit
$ git bisect reset

// How "git bisect run" reads the exit code of your test script (rules from git help bisect).
function verdict(code, firstRun) {
  const c = ((code % 256) + 256) % 256;            // the shell keeps only 8 bits: exit(-1) -> 255
  if (c === 0) return [c, "good (old)"];
  if (c === 125) return [c, "skip - this commit cannot be tested"];
  if (c === 126 || c === 127) return [c, firstRun
    ? "bad? Git first re-runs the script on the good commit; same code there -> abort"
    : "bad (new)"];
  if (c >= 1 && c <= 127) return [c, "bad (new)"];
  return [c, "ABORT the whole bisect (code >= 128, e.g. killed by a signal)"];
}
const cases = [
  [0, "tests passed"], [1, "a test failed"], [2, "pytest: usage error"],
  [124, "timeout(1) gave up"], [125, "build failed, script said skip"],
  [127, "command not found"], [139, "crash: 128 + 11 (SIGSEGV)"], [-1, "exit(-1)"],
];
for (const [code, why] of cases) {
  const [c, v] = verdict(code, code === 127);
  console.log(`${String(code).padStart(4)} -> $?=${String(c).padEnd(4)} ${why.padEnd(31)} ${v}`);
}
// A crash should usually mean "bad", so wrap the program: ./run_tests || exit 1
const wrapped = (code) => (code === 0 ? 0 : code === 125 ? 125 : 1);
console.log("\nwrapped 139 ->", verdict(wrapped(139), false)[1]);

$ git bisect start HEAD v1.0
…
$ git bisect run no-such-tester
running 'no-such-tester'
'no-such-tester': line 1: no-such-tester: command not found
[a122fb65eb5929fb827238c12eb660f083eb7e4f] Add format_taka helper
running 'no-such-tester'
'no-such-tester': line 1: no-such-tester: command not found
error: bogus exit code 127 for good revision
…
$ git bisect reset
…
$ git bisect start HEAD v1.0
…
$ git bisect run sh -c 'exit 200'
running 'sh' '-c' 'exit 200'
error: bisect run failed: exit code 200 from 'sh' '-c' 'exit 200' is < 0 or >= 128

$ git blame -L :print_receipt bill.py
a2aba587 (Rahim  2026-01-15 10:08:00 +0600 27) def print_receipt(cart):
37657780 (Rahim  2026-01-15 10:09:00 +0600 28)     for item, qty in cart:
37657780 (Rahim  2026-01-15 10:09:00 +0600 29)         print(item, qty, format_taka(line_total(item, qty)))
87f84699 (Rahim  2026-01-15 10:13:00 +0600 30)     print("Total:", format_taka(final_total(cart)))
2cd69ab2 (Rahim  2026-01-15 10:19:00 +0600 31)     print("Thank you!")
7331e002 (Nasrin 2026-01-15 10:12:00 +0600 32) 

$ git blame -L 3,5 README.md
^90e700e (Rahim  2026-01-15 10:00:00 +0600 3) A tiny grocery bill calculator for a corner shop.
392acc2c (Nasrin 2026-01-15 10:07:00 +0600 4)
392acc2c (Nasrin 2026-01-15 10:07:00 +0600 5) ## Usage

$ git blame -e -L 12,13 bill.py
40f5f498 (<rahim@example.com> 2026-01-15 10:01:00 +0600 12) def line_total(item, qty):
87f84699 (<rahim@example.com> 2026-01-15 10:13:00 +0600 13)     return PRICES[item] * qty
$ git blame -s -L 12,13 bill.py
40f5f498 12) def line_total(item, qty):
87f84699 13)     return PRICES[item] * qty

$ git blame --porcelain -L 13,13 bill.py
87f846999929544ad01f1b16c309301298516ca3 11 13 1
author Rahim
author-mail <rahim@example.com>
author-time 1768450380
author-tz +0600
committer Rahim
committer-mail <rahim@example.com>
committer-time 1768450380
committer-tz +0600
summary Format code: spaces around operators, double quotes
previous 7331e0026c8ab498791f4f3f8c62dfd007b1da66 bill.py
filename bill.py
	    return PRICES[item] * qty

# A minimal "git blame": each line remembers the commit that last introduced it.
import difflib

versions = [
    ("86bd46b", ["def eid_discount(amount):", "    if amount >= 1000:",
                 "        return amount*0.95", "    return amount"]),
    ("70c0892", ["DISCOUNT_MIN = 1000", "",
                 "def eid_discount(amount):",
                 "    if amount > DISCOUNT_MIN:",
                 "        return amount*0.95",
                 "    return amount"]),
    ("87f8469", ["DISCOUNT_MIN = 1000", "",
                 "def eid_discount(amount):",
                 "    if amount > DISCOUNT_MIN:",
                 "        return amount * 0.95",
                 "    return amount"]),
]

def blame(versions, ignore_space=False):
    key = (lambda s: "".join(s.split())) if ignore_space else (lambda s: s)
    owners, old = [], []
    for commit, new in versions:
        sm = difflib.SequenceMatcher(None, [key(x) for x in old], [key(x) for x in new], autojunk=False)
        fresh = [commit] * len(new)            # unmatched lines belong to this commit
        for tag, i1, i2, j1, j2 in sm.get_opcodes():
            if tag == "equal":                 # unchanged lines keep their old owner
                fresh[j1:j2] = owners[i1:i2]
        owners, old = fresh, new
    return list(zip(owners, old))

for title, flag in (("plain", False), ("ignore whitespace (-w)", True)):
    print(title)
    for n, (c, line) in enumerate(blame(versions, flag), start=2):
        print(f"  {c} {n}) {line}")

$ git blame discount.py
1853d668 (Rahim 2026-01-15 10:14:00 +0600 1) # discount.py - eid discount rules
1853d668 (Rahim 2026-01-15 10:14:00 +0600 2) DISCOUNT_MIN = 1000
1853d668 (Rahim 2026-01-15 10:14:00 +0600 3)
1853d668 (Rahim 2026-01-15 10:14:00 +0600 4) def eid_discount(amount):
1853d668 (Rahim 2026-01-15 10:14:00 +0600 5)     if amount > DISCOUNT_MIN:
1853d668 (Rahim 2026-01-15 10:14:00 +0600 6)         return amount * 0.95
1853d668 (Rahim 2026-01-15 10:14:00 +0600 7)     return amount
$ git blame -C discount.py
1853d668 discount.py (Rahim  2026-01-15 10:14:00 +0600 1) # discount.py - eid discount rules
70c0892d bill.py     (Rahim  2026-01-15 10:11:00 +0600 2) DISCOUNT_MIN = 1000
70c0892d bill.py     (Rahim  2026-01-15 10:11:00 +0600 3)
86bd46bc bill.py     (Nasrin 2026-01-15 10:03:00 +0600 4) def eid_discount(amount):
70c0892d bill.py     (Rahim  2026-01-15 10:11:00 +0600 5)     if amount > DISCOUNT_MIN:
87f84699 bill.py     (Rahim  2026-01-15 10:13:00 +0600 6)         return amount * 0.95
86bd46bc bill.py     (Nasrin 2026-01-15 10:03:00 +0600 7)     return amount
$ git blame -C -w discount.py
…
86bd46bc bill.py     (Nasrin 2026-01-15 10:03:00 +0600 6)         return amount * 0.95
…

$ git blame -C -L 5,5 discount.py
1853d668 (Rahim 2026-01-15 10:14:00 +0600 5)     if amount > DISCOUNT_MIN:
$ git blame -C -L :eid_discount discount.py
86bd46bc bill.py (Nasrin 2026-01-15 10:03:00 +0600 4) def eid_discount(amount):
70c0892d bill.py (Rahim  2026-01-15 10:11:00 +0600 5)     if amount > DISCOUNT_MIN:
87f84699 bill.py (Rahim  2026-01-15 10:13:00 +0600 6)         return amount * 0.95
86bd46bc bill.py (Nasrin 2026-01-15 10:03:00 +0600 7)     return amount

$ git blame -L 12,16 bill.py
40f5f498 (Rahim 2026-01-15 10:01:00 +0600 12) def line_total(item, qty):
87f84699 (Rahim 2026-01-15 10:13:00 +0600 13)     return PRICES[item] * qty
40f5f498 (Rahim 2026-01-15 10:01:00 +0600 14)
05792cac (Rahim 2026-01-15 10:16:00 +0600 15) def format_taka(amount):
05792cac (Rahim 2026-01-15 10:16:00 +0600 16)     return "Tk " + format(amount, ",.2f")
$ git blame -M -L 12,16 bill.py
40f5f498 (Rahim 2026-01-15 10:01:00 +0600 12) def line_total(item, qty):
87f84699 (Rahim 2026-01-15 10:13:00 +0600 13)     return PRICES[item] * qty
40f5f498 (Rahim 2026-01-15 10:01:00 +0600 14)
a122fb65 (Rahim 2026-01-15 10:05:00 +0600 15) def format_taka(amount):
87f84699 (Rahim 2026-01-15 10:13:00 +0600 16)     return "Tk " + format(amount, ",.2f")

$ git log --oneline -S'>= 1000'
70c0892 Use a constant for the discount threshold (#17)
86bd46b Add eid discount function
$ git log --oneline -S'DISCOUNT_MIN'
1853d66 Move eid discount into discount.py
70c0892 Use a constant for the discount threshold (#17)

$ git log --oneline -S'def format_taka'
a122fb6 Add format_taka helper
$ git log --oneline -G'def format_taka'
05792ca Move format_taka next to line_total
a122fb6 Add format_taka helper
$ git log --oneline -S'0.95'
1853d66 Move eid discount into discount.py
86bd46b Add eid discount function
$ git log --oneline -G'0\.95'
1853d66 Move eid discount into discount.py
87f8469 Format code: spaces around operators, double quotes
86bd46b Add eid discount function

// Model of git log -S (count of a string changes) vs -G (a changed line matches a regex).
const history = [
  ["c1 add helper",  ["def line_total():", "def format_taka():", "def bill_total():"]],
  ["c2 move helper", ["def format_taka():", "def line_total():", "def bill_total():"]],
  ["c3 discount",    ["def format_taka():", "def line_total():", "def bill_total():", "x = a*0.95"]],
  ["c4 reformat",    ["def format_taka():", "def line_total():", "def bill_total():", "x = a * 0.95"]],
];
function diffLines(a, b) {                      // LCS table, then walk it: returns changed lines
  const L = Array.from({ length: a.length + 1 }, () => Array(b.length + 1).fill(0));
  for (let i = a.length - 1; i >= 0; i--)
    for (let j = b.length - 1; j >= 0; j--)
      L[i][j] = a[i] === b[j] ? L[i + 1][j + 1] + 1 : Math.max(L[i + 1][j], L[i][j + 1]);
  const out = []; let i = 0, j = 0;
  while (i < a.length || j < b.length) {
    if (i < a.length && j < b.length && a[i] === b[j]) { i++; j++; }
    else if (j < b.length && (i === a.length || L[i][j + 1] >= L[i + 1][j])) out.push("+" + b[j++]);
    else out.push("-" + a[i++]);
  }
  return out.sort((x, y) => (x[0] === "-" ? 0 : 1) - (y[0] === "-" ? 0 : 1));   // "-" first, like Git
}
const count = (lines, s) => lines.join("\n").split(s).length - 1;
function search(s, re) {
  let prev = [];
  for (const [name, lines] of history) {
    const changed = diffLines(prev, lines);
    const S = count(prev, s) !== count(lines, s);
    const G = changed.some((l) => re.test(l.slice(1)));
    console.log(`${name.padEnd(15)} count ${count(prev, s)}->${count(lines, s)}  -S:${S ? "yes" : "no "}` +
      `  -G:${G ? "yes" : "no "}  diff: ${changed.join(" ")}`);
    prev = lines;
  }
}
console.log('-S"format_taka" vs -G"format_taka"'); search("format_taka", /format_taka/);
console.log('\n-S"0.95" vs -G"0\\.95"'); search("0.95", /0\.95/);

$ git log --oneline --stat -S'from discount'
1853d66 Move eid discount into discount.py
 bill.py | 9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)
$ git log --oneline --stat -S'from discount' --pickaxe-all
1853d66 Move eid discount into discount.py
 bill.py     | 9 ++-------
 discount.py | 7 +++++++
 2 files changed, 9 insertions(+), 7 deletions(-)

$ git log --oneline -L :print_receipt:bill.py
2cd69ab Add a thank-you line to the receipt
…
87f8469 Format code: spaces around operators, double quotes
…
7331e00 Add delivery charge
…
3765778 Finish receipt printing
…
a2aba58 WIP: start receipt printing
…

$ git grep -n 1000
discount.py:2:DISCOUNT_MIN = 1000
$ git grep -n 1000 v1.0
v1.0:bill.py:20:    if amount >= 1000:
$ git grep -n -p 0.95
discount.py=4=def eid_discount(amount):
discount.py:6:        return amount * 0.95
$ git grep -n -e amount --and -e 0.95
discount.py:6:        return amount * 0.95
$ git grep -c "def " v1.0 HEAD
v1.0:bill.py:6
HEAD:bill.py:7
HEAD:discount.py:1
HEAD:test_bill.py:1

$ git log --oneline --since=2026-01-15T10:10 --until=2026-01-15T10:14 -- bill.py
1853d66 Move eid discount into discount.py
87f8469 Format code: spaces around operators, double quotes
7331e00 Add delivery charge
70c0892 Use a constant for the discount threshold (#17)
$ git shortlog -sn HEAD -- bill.py
    10	Rahim
     3	Nasrin