Merge আর conflict — fast-forward, three-way merge আর conflict সমাধান

$ git switch main          # যে branch-এ কাজ আনবেন, আগে সেখানে যান
$ git merge feature/x      # feature/x-এর কাজ main-এ আনুন

$ git switch -c feature/eid-discount
Switched to a new branch 'feature/eid-discount'
# (edit shop.conf: discount = 0 -> 10; git add shop.conf)
$ git commit -m "Set eid discount to 10%"
[feature/eid-discount b922684] Set eid discount to 10%
 1 file changed, 1 insertion(+), 1 deletion(-)
# (append a line to README.md; git add README.md)
$ git commit -m "Document eid discount"
[feature/eid-discount f424824] Document eid discount
 1 file changed, 1 insertion(+)
$ git log --oneline --graph --all
* f424824 Document eid discount
* b922684 Set eid discount to 10%
* d273b9b Add README
* 8373eb1 Add shop config
$ git switch main
Switched to branch 'main'
$ git merge-base main feature/eid-discount
d273b9b3e1ea352fd7ce325bf2fdd2602986f09f
$ git rev-parse --short main
d273b9b
$ git merge feature/eid-discount
Updating d273b9b..f424824
Fast-forward
 README.md | 1 +
 shop.conf | 2 +-
 2 files changed, 2 insertions(+), 1 deletion(-)
$ git log --oneline --graph
* f424824 Document eid discount
* b922684 Set eid discount to 10%
* d273b9b Add README
* 8373eb1 Add shop config

$ git switch -c feature/wallet
Switched to a new branch 'feature/wallet'
# (create payment.conf; git add payment.conf)
$ git commit -m "Add mobile wallet payment"
[feature/wallet dcebc15] Add mobile wallet payment
 1 file changed, 1 insertion(+)
 create mode 100644 payment.conf
$ git switch main
Switched to branch 'main'
$ git merge --no-ff feature/wallet
Merge made by the 'ort' strategy.
 payment.conf | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 payment.conf
$ git log --oneline --graph
*   d08a9a3 Merge branch 'feature/wallet'
|\  
| * dcebc15 Add mobile wallet payment
|/  
* f424824 Document eid discount
* b922684 Set eid discount to 10%
* d273b9b Add README
* 8373eb1 Add shop config
$ git cat-file -p HEAD
tree e00a4f5ffb4807231885c1230ceeefd3a2d901fc
parent f424824320ab1c81cdac409b4adbbd257063a918
parent dcebc15ce21e1765da497e38129da947e853163d
author Rahim <rahim@example.com> 1768450020 +0600
committer Rahim <rahim@example.com> 1768450020 +0600

Merge branch 'feature/wallet'

// A commit DAG: each commit lists its parents (a merge commit has two).
const parents = {
  A: [], B: ["A"], C: ["B"], D: ["C"],   // main:    A - B - C - D
  E: ["B"], F: ["E"],                    // feature: branched at B, then E, F
  G: ["D"], H: ["G"],                    // hotfix:  branched at D, then G, H
  M: ["D", "F"],                         // merge commit: first parent D, second F
  P: ["M", "H"], Q: ["H", "M"]           // criss-cross: two merges in opposite directions
};
function ancestors(c) {                  // BFS over parent links; includes c itself
  const seen = new Set([c]), queue = [c];
  while (queue.length)
    for (const p of parents[queue.shift()])
      if (!seen.has(p)) { seen.add(p); queue.push(p); }
  return seen;
}
function mergeBases(x, y) {              // best common ancestors (lowest in the DAG)
  const ax = ancestors(x), ay = ancestors(y);
  const common = [...ax].filter(c => ay.has(c));
  return common.filter(c => !common.some(d => d !== c && ancestors(d).has(c)));
}
function decide(ours, theirs) {
  const bases = mergeBases(ours, theirs);
  let kind;
  if (bases.includes(theirs)) kind = "already up to date";
  else if (bases.includes(ours)) kind = "fast-forward: move pointer " + ours + " -> " + theirs;
  else kind = "three-way merge, new commit with parents " + ours + " and " + theirs;
  return "base " + bases.join(",").padEnd(4) + "| " + kind;
}
for (const [o, t] of [["D", "H"], ["H", "D"], ["D", "F"], ["M", "H"], ["P", "Q"]])
  console.log(("ours " + o + ", theirs " + t).padEnd(18) + decide(o, t));
console.log("common ancestors of D and F:", [...ancestors("D")].filter(c => ancestors("F").has(c)).join(" "));

$ git switch -c feature/free-delivery
Switched to a new branch 'feature/free-delivery'
# (edit shop.conf: free_over = 1000 -> 800; git add)
$ git commit -m "Lower free delivery limit to 800"
[feature/free-delivery 76d3dca] Lower free delivery limit to 800
 1 file changed, 1 insertion(+), 1 deletion(-)
$ git switch main
Switched to branch 'main'
# (edit shop.conf: name = Bazar Mart -> Bazar Mart Online; git add)
$ git commit -m "Rename shop"
[main ec81376] Rename shop
 1 file changed, 1 insertion(+), 1 deletion(-)
$ git log --oneline --graph --all
* ec81376 Rename shop
| * 76d3dca Lower free delivery limit to 800
|/  
*   d08a9a3 Merge branch 'feature/wallet'
|\  
| * dcebc15 Add mobile wallet payment
|/  
* f424824 Document eid discount
* b922684 Set eid discount to 10%
* d273b9b Add README
* 8373eb1 Add shop config
$ git merge-base main feature/free-delivery
d08a9a3cfc64ad87406e4e776dde7a863b121355
$ git merge --ff-only feature/free-delivery
hint: Diverging branches can't be fast-forwarded, you need to either:
hint:
hint: 	git merge --no-ff
hint:
hint: or:
hint:
hint: 	git rebase
hint:
hint: Disable this message with "git config set advice.diverging false"
fatal: Not possible to fast-forward, aborting.
$ git merge feature/free-delivery
Auto-merging shop.conf
Merge made by the 'ort' strategy.
 shop.conf | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
$ cat shop.conf
name = Bazar Mart Online
currency = BDT
delivery_fee = 60
free_over = 800
discount = 10
$ git log --oneline --graph -6
*   2693581 Merge branch 'feature/free-delivery'
|\  
| * 76d3dca Lower free delivery limit to 800
* | ec81376 Rename shop
|/  
*   d08a9a3 Merge branch 'feature/wallet'
|\  
| * dcebc15 Add mobile wallet payment
|/  
* f424824 Document eid discount

$ git merge other
Auto-merging f.txt
CONFLICT (content): Merge conflict in f.txt
Automatic merge failed; fix conflicts and then commit the result.
$ cat f.txt
a = 1
<<<<<<< HEAD
b = 20
c = 3
=======
b = 2
c = 30
>>>>>>> other

// Line-based three-way merge: base, ours, theirs -> merged lines or conflict markers.
function match(a, b) {                   // LCS: m[i] = index in b of line a[i], or -1
  const L = Array.from({ length: a.length + 1 }, () => new 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 m = new Array(a.length).fill(-1);
  for (let i = 0, j = 0; i < a.length && j < b.length; )
    if (a[i] === b[j]) m[i++] = j++; else if (L[i + 1][j] >= L[i][j + 1]) i++; else j++;
  return m;
}
function merge3(base, ours, theirs, name) {
  const mo = match(base, ours), mt = match(base, theirs), out = [], same = (x, y) => x.join() === y.join();
  let i = 0, io = 0, it = 0, conflicts = 0;
  for (;;) {
    if (i < base.length && mo[i] === io && mt[i] === it) { out.push(base[i]); i++; io++; it++; continue; }
    let j = i;                           // next base line kept by BOTH sides = sync point
    while (j < base.length && (mo[j] < 0 || mt[j] < 0)) j++;
    const eo = j < base.length ? mo[j] : ours.length, et = j < base.length ? mt[j] : theirs.length;
    const b = base.slice(i, j), o = ours.slice(io, eo), t = theirs.slice(it, et);
    if (!b.length && !o.length && !t.length) break;
    if (same(o, b)) out.push(...t);      // only theirs changed this hunk
    else if (same(t, b) || same(o, t)) out.push(...o);   // only ours, or both identical
    else { conflicts++; out.push("<<<<<<< HEAD", ...o, "=======", ...t, ">>>>>>> " + name); }
    i = j; io = eo; it = et;
  }
  console.log("--- merge " + name + ": " + (conflicts ? conflicts + " conflict(s)" : "clean")); console.log(out.join("\n"));
}
const base = ["name = Bazar Mart", "currency = BDT", "delivery_fee = 60", "free_over = 1000", "discount = 10"];
const ours = ["name = Bazar Mart Online", ...base.slice(1)];
merge3(base, ours, base.map(l => l.replace("1000", "800")), "feature/free-delivery");
merge3(base, ours.map(l => l.replace("= 60", "= 70")), base.map(l => l.replace("= 60", "= 50")), "hotfix/delivery");

$ git switch -c hotfix/delivery
Switched to a new branch 'hotfix/delivery'
# (edit shop.conf: delivery_fee = 60 -> 50; git add)
$ git commit -m "Cut delivery fee to 50"
[hotfix/delivery 2960209] Cut delivery fee to 50
 1 file changed, 1 insertion(+), 1 deletion(-)
$ git switch main
Switched to branch 'main'
# (edit shop.conf: delivery_fee = 60 -> 70; git add)
$ git commit -m "Raise delivery fee to 70"
[main 5814e0c] Raise delivery fee to 70
 1 file changed, 1 insertion(+), 1 deletion(-)
$ git merge hotfix/delivery
Auto-merging shop.conf
CONFLICT (content): Merge conflict in shop.conf
Automatic merge failed; fix conflicts and then commit the result.
$ git status
On branch main
You have unmerged paths.
  (fix conflicts and run "git commit")
  (use "git merge --abort" to abort the merge)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   shop.conf

no changes added to commit (use "git add" and/or "git commit -a")
$ cat shop.conf
name = Bazar Mart Online
currency = BDT
<<<<<<< HEAD
delivery_fee = 70
=======
delivery_fee = 50
>>>>>>> hotfix/delivery
free_over = 800
discount = 10

$ git merge cleanup
Auto-merging banner.txt
CONFLICT (add/add): Merge conflict in banner.txt
CONFLICT (modify/delete): coupon.txt deleted in cleanup and modified in HEAD.  Version HEAD of coupon.txt left in tree.
Automatic merge failed; fix conflicts and then commit the result.
$ git status
On branch main
You have unmerged paths.
  (fix conflicts and run "git commit")
  (use "git merge --abort" to abort the merge)

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	both added:      banner.txt
	deleted by them: coupon.txt

no changes added to commit (use "git add" and/or "git commit -a")
$ cat banner.txt
<<<<<<< HEAD
banner from main
=======
banner from cleanup
>>>>>>> cleanup

$ git config merge.conflictStyle zdiff3
$ git merge hotfix/delivery
Auto-merging shop.conf
CONFLICT (content): Merge conflict in shop.conf
Automatic merge failed; fix conflicts and then commit the result.
$ cat shop.conf
name = Bazar Mart Online
currency = BDT
<<<<<<< HEAD
delivery_fee = 70
||||||| 2693581
delivery_fee = 60
=======
delivery_fee = 50
>>>>>>> hotfix/delivery
free_over = 800
discount = 10

$ git -c merge.conflictStyle=diff3 merge theirs
Auto-merging menu.txt
CONFLICT (content): Merge conflict in menu.txt
Automatic merge failed; fix conflicts and then commit the result.
$ cat menu.txt
title = Menu
<<<<<<< HEAD
rice = 55
note = prices from Jan
tea = 12
||||||| 6d28d02
rice = 50
tea = 10
=======
rice = 55
note = prices from Jan
tea = 15
>>>>>>> theirs
end
$ git merge --abort
$ git -c merge.conflictStyle=zdiff3 merge theirs
Auto-merging menu.txt
CONFLICT (content): Merge conflict in menu.txt
Automatic merge failed; fix conflicts and then commit the result.
$ cat menu.txt
title = Menu
rice = 55
note = prices from Jan
<<<<<<< HEAD
tea = 12
||||||| 6d28d02
rice = 50
tea = 10
=======
tea = 15
>>>>>>> theirs
end

$ git diff
diff --cc shop.conf
index 1464268,1f0a4cd..0000000
--- a/shop.conf
+++ b/shop.conf
@@@ -1,5 -1,5 +1,9 @@@
  name = Bazar Mart Online
  currency = BDT
++<<<<<<< HEAD
 +delivery_fee = 70
++=======
+ delivery_fee = 50
++>>>>>>> hotfix/delivery
  free_over = 800
  discount = 10
$ git log --merge --oneline
5814e0c Raise delivery fee to 70
2960209 Cut delivery fee to 50
$ git ls-files -u
100644 960fa0af662d03fdea3bf18a4dd36bdb1108e033 1	shop.conf
100644 14642685fe346b8302320db59940606009daee46 2	shop.conf
100644 1f0a4cde2aab4d5f81a2ff7c4336d4c7a93069e7 3	shop.conf
$ git show :1:shop.conf
name = Bazar Mart Online
currency = BDT
delivery_fee = 60
free_over = 800
discount = 10
$ git merge --abort
$ git status --short

# (edit shop.conf: keep only "delivery_fee = 50", remove the markers)
$ git add shop.conf
$ git status
On branch main
All conflicts fixed but you are still merging.
  (use "git commit" to conclude merge)

Changes to be committed:
	modified:   shop.conf
$ git merge --continue
[main 82b90b1] Merge branch 'hotfix/delivery'

$ git checkout --theirs banner.txt
Updated 1 path from the index
$ cat banner.txt
banner from cleanup
$ git add banner.txt
$ git rm coupon.txt
rm 'coupon.txt'
$ git status --short
M  banner.txt
D  coupon.txt
$ git commit -m "Merge cleanup: drop coupon, take their banner"
[main d13d4c1] Merge cleanup: drop coupon, take their banner

$ git merge feature/invoice
Auto-merging bill.js
Merge made by the 'ort' strategy.
 bill.js | 1 +
 1 file changed, 1 insertion(+)
$ git grep -nw fee
bill.js:5:function invoiceLine(total) { return "Delivery: " + fee(total); }

// Semantic conflict: two clean, separate edits that break the program only together.
const base = [
  'function fee(total) { return total >= 1000 ? 0 : 60; }',
  'function checkout(total) { return total + fee(total); }',
  '// ---- invoice helpers ----',
  'function invoiceTitle() { return "Bazar Mart"; }'];
// main renames fee -> deliveryFee (changes lines 1-2 only)
const ours = base.map(l => l.replace(/\bfee\(/g, "deliveryFee("));
// feature adds a NEW caller of fee() after line 4 (an insertion only)
const added = 'function invoiceLine(total) { return "Delivery: " + fee(total); }';
const theirs = [...base, added];
// The two hunks touch lines 1-2 and "after 4": separated by unchanged lines -> no textual conflict.
const merged = [...ours, added];
function test(label, lines) {            // a tiny test runner
  const results = [];
  const check = (name, fn) => {
    try { const ok = fn(); results.push((ok ? "PASS " : "FAIL ") + name); }
    catch (e) { results.push("ERROR " + name + ": " + e.name + ": " + e.message); }
  };
  const api = new Function(lines.join("\n") +
    "\nreturn { checkout, invoiceLine: typeof invoiceLine === 'function' ? invoiceLine : null };")();
  check("checkout(500) is 560", () => api.checkout(500) === 560);
  if (api.invoiceLine) check("invoiceLine(500) shows 60", () => api.invoiceLine(500) === "Delivery: 60");
  console.log(label.padEnd(8) + results.join(" | "));
}
test("base", base);
test("ours", ours);
test("theirs", theirs);
test("merged", merged);
console.log("textual conflict markers in merged file:", merged.some(l => l.startsWith("<<<<<<<")));

// How often do two branches conflict? Each branch rewrites k random lines of a
// 300-line file. As in Git, a conflict happens when the two sides change the same
// line or two neighbouring lines (no unchanged line between their hunks).
function mulberry32(a) { return () => { 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 LINES = 300, TRIALS = 2000;
function pick(rnd, k) {                  // k distinct random line numbers
  const s = new Set();
  while (s.size < k) s.add(Math.floor(rnd() * LINES));
  return s;
}
function conflictRate(k) {
  const rnd = mulberry32(2026);
  let hits = 0;
  for (let n = 0; n < TRIALS; n++) {
    const ours = pick(rnd, k), theirs = pick(rnd, k);
    let clash = false;
    for (const i of ours) if (theirs.has(i) || theirs.has(i - 1) || theirs.has(i + 1)) { clash = true; break; }
    if (clash) hits++;
  }
  return hits / TRIALS;
}
console.log("lines changed per branch   P(conflict)");
for (const k of [1, 3, 5, 10, 20, 40])
  console.log(String(k).padStart(12) + (100 * conflictRate(k)).toFixed(1).padStart(22) + "%");

$ git log --oneline --graph
*   82b90b1 Merge branch 'hotfix/delivery'
|\  
| * 2960209 Cut delivery fee to 50
* | 5814e0c Raise delivery fee to 70
|/  
*   2693581 Merge branch 'feature/free-delivery'
|\  
| * 76d3dca Lower free delivery limit to 800
* | ec81376 Rename shop
|/  
*   d08a9a3 Merge branch 'feature/wallet'
|\  
| * dcebc15 Add mobile wallet payment
|/  
* f424824 Document eid discount
* b922684 Set eid discount to 10%
* d273b9b Add README
* 8373eb1 Add shop config
$ git log --oneline --first-parent
82b90b1 Merge branch 'hotfix/delivery'
5814e0c Raise delivery fee to 70
2693581 Merge branch 'feature/free-delivery'
ec81376 Rename shop
d08a9a3 Merge branch 'feature/wallet'
f424824 Document eid discount
b922684 Set eid discount to 10%
d273b9b Add README
8373eb1 Add shop config
$ git log --oneline --merges
82b90b1 Merge branch 'hotfix/delivery'
2693581 Merge branch 'feature/free-delivery'
d08a9a3 Merge branch 'feature/wallet'