| 1 | /* |
| 2 | * tdq.c: implement a 'to-do queue', a simple de-duplicating to-do |
| 3 | * list mechanism. |
| 4 | */ |
| 5 | |
| 6 | #include <assert.h> |
| 7 | |
| 8 | #include "puzzles.h" |
| 9 | |
| 10 | /* |
| 11 | * Implementation: a tdq consists of a circular buffer of size n |
| 12 | * storing the integers currently in the queue, plus an array of n |
| 13 | * booleans indicating whether each integer is already there. |
| 14 | * |
| 15 | * Using a circular buffer of size n to store between 0 and n items |
| 16 | * inclusive has an obvious failure mode: if the input and output |
| 17 | * pointers are the same, how do you know whether that means the |
| 18 | * buffer is full or empty? |
| 19 | * |
| 20 | * In this application we have a simple way to tell: in the former |
| 21 | * case, the flags array is all 1s, and in the latter case it's all |
| 22 | * 0s. So we could spot that case and check, say, flags[0]. |
| 23 | * |
| 24 | * However, it's even easier to simply determine whether the queue is |
| 25 | * non-empty by testing flags[buffer[op]] - that way we don't even |
| 26 | * _have_ to compare ip against op. |
| 27 | */ |
| 28 | |
| 29 | struct tdq { |
| 30 | int n; |
| 31 | int *queue; |
| 32 | int ip, op; /* in pointer, out pointer */ |
| 33 | char *flags; |
| 34 | }; |
| 35 | |
| 36 | tdq *tdq_new(int n) |
| 37 | { |
| 38 | int i; |
| 39 | tdq *tdq = snew(struct tdq); |
| 40 | tdq->queue = snewn(n, int); |
| 41 | tdq->flags = snewn(n, char); |
| 42 | for (i = 0; i < n; i++) { |
| 43 | tdq->queue[i] = 0; |
| 44 | tdq->flags[i] = 0; |
| 45 | } |
| 46 | tdq->n = n; |
| 47 | tdq->ip = tdq->op = 0; |
| 48 | return tdq; |
| 49 | } |
| 50 | |
| 51 | void tdq_free(tdq *tdq) |
| 52 | { |
| 53 | sfree(tdq->queue); |
| 54 | sfree(tdq->flags); |
| 55 | sfree(tdq); |
| 56 | } |
| 57 | |
| 58 | void tdq_add(tdq *tdq, int k) |
| 59 | { |
| 60 | assert((unsigned)k < (unsigned)tdq->n); |
| 61 | if (!tdq->flags[k]) { |
| 62 | tdq->queue[tdq->ip] = k; |
| 63 | tdq->flags[k] = 1; |
| 64 | if (++tdq->ip == tdq->n) |
| 65 | tdq->ip = 0; |
| 66 | } |
| 67 | } |
| 68 | |
| 69 | int tdq_remove(tdq *tdq) |
| 70 | { |
| 71 | int ret = tdq->queue[tdq->op]; |
| 72 | |
| 73 | if (!tdq->flags[ret]) |
| 74 | return -1; |
| 75 | |
| 76 | tdq->flags[ret] = 0; |
| 77 | if (++tdq->op == tdq->n) |
| 78 | tdq->op = 0; |
| 79 | |
| 80 | return ret; |
| 81 | } |
| 82 | |
| 83 | void tdq_fill(tdq *tdq) |
| 84 | { |
| 85 | int i; |
| 86 | for (i = 0; i < tdq->n; i++) |
| 87 | tdq_add(tdq, i); |
| 88 | } |