Updates on how we generate permutations.
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@ -220,7 +220,7 @@
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"_journal/2024-02/2024-02-27.md": "f75a0d04a875aeee932343dae0c78768",
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"filesystems/index.md": "cbd2b0290a3ba3b32abec4bd8bfefad5",
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"filesystems/cas.md": "34906013a2a60fe5ee0e31809b4838aa",
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"git/objects.md": "b95228a78744d3f9fe173e575aa0445a",
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"git/objects.md": "1c867853b080700a73de55b7be185f8d",
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"git/index.md": "83d2d95fc549d9e8436946c7bd058d15",
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"encoding/integer.md": "84b2ce080e0e756f265257e57467f0e6",
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"_journal/2024-02-29.md": "f610f3caed659c1de3eed5f226cab508",
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@ -237,7 +237,7 @@
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"_journal/2024-03/2024-03-02.md": "8136792b0ee6e08232e4f60c88d461d2",
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"_journal/2024-03-04.md": "9ec052061e7a613ff877a4488576e82f",
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"_journal/2024-03/2024-03-03.md": "64e2f17b4d57a6bd42a3d1b7f2851b83",
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"_journal/2024-03-05.md": "a285ac3e48e335c50c42ee20b0cb0472",
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"_journal/2024-03-05.md": "e9a911c19bb4c0ff451db793248cb4bb",
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"_journal/2024-03/2024-03-04.md": "4948d90a08af2cff58c629c9a2e11ee4",
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"algebra/sequences/geometric.md": "53936ec392b3b714bd4a9bdb4554b582",
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"algebra/sequences/arithmetic.md": "80381ca0f2b3b9a1c155c597a7dea75a"
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@ -5,14 +5,24 @@ title: "2024-03-05"
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- [x] Anki Flashcards
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- [x] KoL
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- [ ] Sheet Music (10 min.)
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- [ ] Go (1 Life & Death Problem)
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- [x] Go (1 Life & Death Problem)
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- [ ] Korean (Read 1 Story)
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- [ ] Interview Prep (1 Practice Problem)
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- [ ] Log Work Hours (Max 3 hours)
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- [x] Interview Prep (1 Practice Problem)
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- [x] Log Work Hours (Max 3 hours)
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* Arithmetic and geometric sequences in "Discrete Mathematics: An Open Introduction".
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* Completed chapter exercises 2.2.
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* TODO
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* Finish up tree objects and commit objects.
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* 101weiqi
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* 2 interview problems.
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* Finished adding flashcards for git tree and commit objects.
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* 101weiqi (serial numbers)
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* Q-82289
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* Q-350205
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* Q-77774
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* Q-267476
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* Q-184824
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* Q-102329
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* Q-152376
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* Q-1358
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* Q-114783
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* Q-26296
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* Leetcode Problems
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* [Next Permutation](https://leetcode.com/problems/next-permutation/)
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@ -39,7 +39,7 @@ void combinations(const int n, const int k, int A[static n]) {
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}
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```
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The above approach prints out all $k$-combinations of a given array.
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The above approach prints out all $k$-combinations of an array.
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%%ANKI
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Basic
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@ -12,38 +12,38 @@ tags:
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A **permutation** of some $n$ objects is a (possible) rearrangement of those $n$ objects. The number of permutations is $n!$ since there are $n$ possible ways to pick the first object, $(n - 1)$ possible ways to pick the second, and so on.
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```c
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void permutations_aux(
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const size_t n,
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int A[static n],
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int res[static n],
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uint64_t choices
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) {
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if (!choices) {
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for (size_t i = 0; i < n; ++i) {
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printf("%d ", A[res[i]]);
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void next(const size_t n, int A[static n]) {
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size_t pivot = -1;
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for (size_t i = n - 1; i >= 1; --i) {
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if (A[i - 1] < A[i]) {
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pivot = i - 1;
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break;
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}
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printf("\n");
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}
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if (pivot == -1) {
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reverse(0, n - 1, A);
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return;
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}
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unsigned int weight = n - bit_weight(choices);
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for (unsigned int i = 0; i < 64; ++i) {
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uint64_t next = 1L << i;
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if (choices & next) {
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res[weight] = i;
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permutations_aux(n, A, res, choices & ~next);
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size_t j = pivot;
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for (size_t i = pivot + 1; i < n; ++i) {
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if (A[pivot] < A[i] && (j == pivot || A[i] < A[j])) {
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j = i;
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}
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}
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swap(pivot, j, A);
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reverse(pivot + 1, n - 1, A);
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}
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void permutations(const size_t n, int A[static n]) {
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int *res = malloc(sizeof(int) * n);
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permutations_aux(n, A, res, (1L << n) - 1);
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free(res);
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size_t iters = factorial(n);
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for (size_t i = 0; i < iters; ++i) {
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print_array(n, A);
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next(n, A);
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}
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}
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```
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The above approach prints out all permutations of a given array, provided the array contains at most `64` digits. It relies on `bit_weight` as defined in [[binary/index|binary]].
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The above approach prints out all permutations of an array.
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%%ANKI
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Basic
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@ -44,6 +44,14 @@ Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Soft
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<!--ID: 1709177255589-->
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END%%
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%%ANKI
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Basic
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How is the SHA-1 of a git object generated?
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Back: By calculating the checksum of the object header + contents.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709674569885-->
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END%%
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%%ANKI
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Basic
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What is "oid" an acronym for?
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@ -117,7 +125,7 @@ END%%
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%%ANKI
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Basic
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Where does e.g. `d670460b4b4aece5915caf5c68d12f560a9fe3e4` live in the object database?
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Where does `d670460b4b4aece5915caf5c68d12f560a9fe3e4` live in the object database?
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Back: At `.git/objects/d6/70460b4b4aece5915caf5c68d12f560a9fe3e4`.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709177255668-->
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@ -182,6 +190,14 @@ Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Soft
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<!--ID: 1709345254716-->
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END%%
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%%ANKI
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Basic
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What plumbing command can be used to find the git object type of an oid?
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Back: `cat-file`
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709674569890-->
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END%%
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## Blobs
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The **b**inary **l**arge **ob**ject (blob) is used to represent arbitrary files.
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@ -315,6 +331,14 @@ Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Soft
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<!--ID: 1709345254740-->
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END%%
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%%ANKI
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Basic
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What kind of git objects can a tree contain?
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Back: Blobs and trees.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709674569895-->
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END%%
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%%ANKI
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Cloze
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{`hash-object`} is to blobs as {`write-tree`} is to trees.
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@ -424,6 +448,75 @@ Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Soft
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<!--ID: 1709349279660-->
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END%%
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## Commits
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Commit objects provide metadata information about particular trees.
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> While a tree represents a particular directory state of a working directory, a commit represents that state in "time", and explains how to get there.
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%%ANKI
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Basic
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What distinguishes a tree from a commit?
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Back: A commit contains data about a tree (e.g. author, parent commits, etc.).
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709674569898-->
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END%%
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%%ANKI
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Basic
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How do git commits maintain a traversable history?
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Back: Each commit can have parent commits associated with it.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709674569902-->
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END%%
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%%ANKI
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Basic
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What kind of git object is responsible for maintaining history?
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Back: Commits.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709674569906-->
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END%%
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%%ANKI
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Basic
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What plumbing subcommand is used to create commits?
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Back: `commit-tree`
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709674569911-->
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END%%
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%%ANKI
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Cloze
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{`write-tree`} is to trees as {`commit-tree`} is to commits.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709674569916-->
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END%%
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%%ANKI
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Basic
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What does the decompressed header of a commit object look like?
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Back: `commit <size><NUL>`
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709674569920-->
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END%%
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%%ANKI
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Basic
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Commits points to what kind of git object?
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Back: Trees.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709674569924-->
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END%%
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%%ANKI
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Basic
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What does the `commit-tree` subcommand return?
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Back: The oid of the new commit object.
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Reference: Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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<!--ID: 1709674569928-->
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END%%
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## References
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* Scott Chacon, *Pro Git*, Second edition, The Expert’s Voice in Software Development (New York, NY: Apress, 2014).
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