sapling/linelog/linelog.c

/*
 * Copyright 2016-present Facebook. All Rights Reserved.
 *
 * linelog.c: data structure tracking line changes
 *
 * This software may be used and distributed according to the terms of the
 * GNU General Public License version 2 or any later version.
 */

#include "linelog.h"
#include <assert.h> /* assert */
#include <stdbool.h> /* bool */
#include <stdlib.h> /* realloc, free */
#include <string.h> /* NULL, memcpy, memmove, memset */
#include <arpa/inet.h> /* htonl, ntohl */

/* linelog_buf.data is a plain array of instructions.

   a linelog instruction has 8 bytes:

     opcode:    2 bits (linelog_opcode)
     operand1: 30 bits (linelog_revnum)
     operand2: 32 bits (linelog_offset | linelog_linenum)

   the first 8-byte slot is not a real instruction, but a 32-bit maxrev then
   a 32-bit instruction count indicating used buffer size. it can be parsed as
   a normal instruction to extract the information. the code usually uses
   "inst0" as the variable name for that purpose.

   real instructions start from the 9th byte. */

typedef enum {
	JGE = 0, /* if rev >= operand1, jump to operand2 */
	JL = 1, /* if rev < operand1, jump to operand2 */
	LINE = 2, /* line introduced by rev = operand1, linenum = operand2 */
} linelog_opcode;

typedef struct {
	linelog_opcode opcode;
	linelog_revnum rev;    /* uint32_t operand1 */
	linelog_offset offset; /* uint32_t operand2, linelog_linenum linenum */
} linelog_inst;

/* static assert uint32_t, linelog_{linenum,revnum,offset} have a same size */
extern char linelog_assert_[1 / (sizeof(linelog_revnum) == sizeof(uint32_t))];
extern char linelog_assert_[1 / (sizeof(linelog_linenum) == sizeof(uint32_t))];
extern char linelog_assert_[1 / (sizeof(linelog_offset) == sizeof(uint32_t))];

/* size of the encoded representation, not sizeof(linelog_inst) */
#define INST_SIZE 8

/* like linelog_{offset,linenum} but less likely to overflow */
typedef size_t linelog_loffset;
typedef size_t linelog_llinenum;

/* hard limits, smaller than the physical limits to reserve some bits */
#ifndef MIN
#define MIN(x, y) (((x) < (y)) ? (x) : (y))
#endif
static const linelog_loffset MAX_OFFSET =
	MIN(0x0ffffff0u, SIZE_MAX / INST_SIZE);
static const linelog_llinenum MAX_LINENUM =
	MIN(0x1ffffff0u, SIZE_MAX / sizeof(linelog_lineinfo));
static const linelog_revnum MAX_REVNUM = 0x1ffffff0u;

/* uint8_t[8] -> linelog_inst */
static inline void decode(const uint8_t data[INST_SIZE], linelog_inst *inst) {
	uint32_t buf[2];
	memcpy(buf, data, sizeof(buf));
	buf[0] = ntohl(buf[0]);
	buf[1] = ntohl(buf[1]);
	inst->opcode = (linelog_opcode)(buf[0] & 3);
	inst->rev = (buf[0] >> 2) & 0x3fffffffu;
	inst->offset = buf[1];
}

/* uint8_t[8] <- linelog_inst */
static inline void encode(uint8_t data[INST_SIZE], const linelog_inst *inst) {
	uint32_t buf[2];
	buf[0] = htonl((uint32_t)(inst->opcode) | (inst->rev << 2));
	buf[1] = htonl(inst->offset);
	memcpy(data, buf, sizeof(buf));
}

/* read instruction, with error checks */
static inline linelog_result readinst(const linelog_buf *buf,
		linelog_inst *inst, linelog_loffset offset) {
	if (buf == NULL || buf->data == NULL || buf->size < INST_SIZE
			|| offset >= MAX_OFFSET)
		return LINELOG_RESULT_EILLDATA;
	size_t len = htonl(((const uint32_t *)buf->data)[1]);
	if (len > buf->size / INST_SIZE || offset >= len)
		return LINELOG_RESULT_EILLDATA;
	size_t offsetinbytes = (size_t)offset * INST_SIZE;
	decode(buf->data + offsetinbytes, inst);
	return LINELOG_RESULT_OK;
}

/* write instruction, with error checks */
static inline linelog_result writeinst(linelog_buf *buf,
		const linelog_inst *inst, linelog_loffset offset) {
	if (offset >= MAX_OFFSET)
		return LINELOG_RESULT_EOVERFLOW;
	if (buf == NULL || (buf->data == NULL && buf->size > 0))
		return LINELOG_RESULT_EILLDATA;
	size_t offsetinbytes = (size_t)offset * INST_SIZE;
	if (offsetinbytes + INST_SIZE > buf->size) {
		buf->neededsize = offsetinbytes + INST_SIZE;
		return LINELOG_RESULT_ENEEDRESIZE;
	}
	encode(buf->data + offsetinbytes, inst);
	return LINELOG_RESULT_OK;
}

/* helper to make code shorter */
#define returnonerror(expr) { \
	linelog_result result = (expr); \
	if (result != LINELOG_RESULT_OK) \
		return result; \
}
#define mustsuccess(expr) { \
	linelog_result result = (expr); \
	(void)result; /* eliminate "unused" warning with NDEBUG set */ \
	assert(result == LINELOG_RESULT_OK); \
}

/* ensure `ar->lines[0:linecount]` are valid */
static linelog_result reservelines(linelog_annotateresult *ar,
		linelog_llinenum linecount) {
	if (linecount >= MAX_LINENUM)
		return LINELOG_RESULT_EOVERFLOW;
	if (ar->maxlinecount < linecount) {
		size_t size = sizeof(linelog_lineinfo) * linecount;
		void *p = realloc(ar->lines, size);
		if (p == NULL)
			return LINELOG_RESULT_ENOMEM;
		ar->lines = (linelog_lineinfo *)p;
		ar->maxlinecount = (linelog_linenum)linecount;
	}
	return LINELOG_RESULT_OK;
}

/* APIs declared in .h */

void linelog_annotateresult_clear(linelog_annotateresult *ar) {
	free(ar->lines);
	memset(ar, 0, sizeof(linelog_annotateresult));
}

linelog_result linelog_clear(linelog_buf *buf) {
	linelog_inst insts[2] = { { .offset = 2 }, { .offset = 0 } };
	returnonerror(writeinst(buf, &insts[1], 1));
	returnonerror(writeinst(buf, &insts[0], 0));
	return LINELOG_RESULT_OK;
}

size_t linelog_getactualsize(const linelog_buf *buf) {
	linelog_inst inst0;
	linelog_result r = readinst(buf, &inst0, 0);
	if (r != LINELOG_RESULT_OK)
		return 0;
	return (size_t)(inst0.offset) * INST_SIZE;
}

linelog_revnum linelog_getmaxrev(const linelog_buf *buf) {
	linelog_inst inst0;
	linelog_result r = readinst(buf, &inst0, 0);
	if (r != LINELOG_RESULT_OK)
		return 0;
	return inst0.rev;
}

inline static linelog_result appendline(linelog_annotateresult *ar,
		const linelog_inst *inst, linelog_offset offset) {
	linelog_lineinfo info = { .rev = inst ? inst->rev : 0,
		.linenum = inst ? inst->offset /* linenum */ : 0,
		.offset = offset };
	returnonerror(reservelines(ar, ar->linecount + 1));
	ar->lines[ar->linecount++] = info;
	return LINELOG_RESULT_OK;
}

linelog_result linelog_annotate(const linelog_buf *buf,
		 linelog_annotateresult *ar, linelog_revnum rev) {
	linelog_inst inst0;
	returnonerror(readinst(buf, &inst0, 0));

	linelog_offset pc, nextpc = 1, endoffset = 0;
	ar->linecount = 0;
	size_t step = (size_t)inst0.offset;

	while ((pc = nextpc++) != 0 && --step) {
		linelog_inst i;
		returnonerror(readinst(buf, &i, pc));

		switch (i.opcode) {
		case JGE: case JL: /* conditional jump */
			if (i.opcode == JGE ? rev >= i.rev : rev < i.rev) {
				nextpc = i.offset;
				if (nextpc == 0) /* met the END marker */
					endoffset = pc;
			}
			break;
		case LINE: /* append a line */
			returnonerror(appendline(ar, &i, pc));
			break;
		default: /* unknown opcode */
			return LINELOG_RESULT_EILLDATA;
		}
	}

	if (endoffset == 0) /* didn't meet a valid END marker */
		return LINELOG_RESULT_EILLDATA;

	/* ar->lines[ar->linecount].offset records the endoffset */
	returnonerror(appendline(ar, NULL, endoffset));
	ar->linecount--; /* do not include this special line */
	return LINELOG_RESULT_OK;
}

static linelog_result replacelines(linelog_buf *buf, linelog_annotateresult *ar,
		linelog_revnum brev, linelog_linenum a1, linelog_linenum a2,
		linelog_linenum b1, linelog_linenum b2,
		const linelog_revnum *brevs, const linelog_linenum *blinenums) {
	/*       buf   before     after
	               --------   --------
	               ....       ....
	      a1addr > (a1inst)   JGE     0 oldlen   [5]
	    a1addr+1 > ...        ...
	               ....       ....
	      a2addr > ...        ...
	               ....       ....
	      oldlen > (end)      JL   brev pjge     [1]
	                          LINE brev b1       [1]
	                          LINE brev b1+1     [1]
	                          ....               [1]
	                          LINE brev b2-1     [1]
	        pjge >            JGE  brev a2addr   [2]
	   a1newaddr >            (a1inst)           [3]
	                          JGE     0 a1addr+1 [4]
	      newlen >            (end)

	   [1]: insert new lines. only exist if b1 < b2
	   [2]: delete old lines. only exist if a1 < a2
	   [3]: move a1inst to new place, as it will be rewritten in [5]
	   [4]: jump back. only exist if a1inst is not an unconditional jump
	   [5]: rewrite the old instruction to jump to the new block */

	/* sanity check */
	linelog_inst inst0;
	returnonerror(readinst(buf, &inst0, 0));
	if (brev >= MAX_REVNUM || a2 >= MAX_LINENUM || b2 >= MAX_LINENUM)
		return LINELOG_RESULT_EOVERFLOW;
	if (a2 < a1 || b2 < b1 || !ar || a2 > ar->linecount || brev == 0
			|| ar->linecount >= ar->maxlinecount)
		return LINELOG_RESULT_EILLDATA;

	/* useful variables for both step I and III */
	linelog_offset oldlen = inst0.offset;
	linelog_offset a1addr = ar->lines[a1].offset;
	linelog_inst a1inst;
	returnonerror(readinst(buf, &a1inst, a1addr));
	bool a1instisjge0 = (a1inst.opcode == JGE && a1inst.rev == 0);

	/* step I: reserve size for buf: (newlen - oldlen) more instructions */
	linelog_loffset newlen = (linelog_loffset)oldlen
		+ (b2 - b1 /* LINE */ + (b2 > b1) /* JL brev */) /* [1] */
		+ (a2 > a1) /* JGE brev */                       /* [2] */
		+ 1 /* a1inst */                                 /* [3] */
		+ (a1instisjge0 ? 0 : 1) /* JGE 0  */            /* [4] */;
	if (newlen >= MAX_OFFSET)
		return LINELOG_RESULT_EOVERFLOW;
	size_t neededsize = (size_t)newlen * INST_SIZE;
	if (neededsize > buf->size) {
		buf->neededsize = neededsize;
		return LINELOG_RESULT_ENEEDRESIZE;
	}

	/* step II: reserve space for annotateresult */
	linelog_llinenum newlinecount =
		(linelog_llinenum)ar->linecount + b2 - b1 - (a2 - a1);
	returnonerror(reservelines(ar, newlinecount + 1));
	assert(ar->linecount < ar->maxlinecount);

	/* writeinst should not fail for remaining steps - we have reserved
	   enough space. any failure will be a huge headache for the caller. */

	/* step III: update linelog_buf */
	#define appendinst(inst) \
		mustsuccess(writeinst(buf, &inst, inst0.offset++));
	if (b1 < b2) { /* [1] */
		linelog_offset pjge = oldlen + (b2 - b1 + 1);
		linelog_inst jl = { .opcode = JL, .rev = brev, .offset = pjge };
		appendinst(jl);
		for (linelog_linenum i = b1; i < b2; ++i) {
			linelog_inst lineinst = { .opcode = LINE,
				.rev = brevs ? brevs[i] : brev,
				.offset /* linenum */ =
					blinenums ? blinenums[i] : i };
			appendinst(lineinst);
		}
	}
	if (a1 < a2) { /* [2] */
		linelog_offset a2addr = ar->lines[a2].offset;
		/* delete a chunk of an old commit. be conservative, do not
		   touch invisible lines between a2 - 1 and a2 */
		if (a2 > 0 && brev < inst0.rev /* maxrev */)
			a2addr = ar->lines[a2 - 1].offset + 1;
		linelog_inst jge = { .opcode = JGE, .rev = brev,
			.offset = a2addr };
		appendinst(jge);
	}
	linelog_offset a1newaddr = inst0.offset;
	appendinst(a1inst); /* [3] */
	if (!a1instisjge0) { /* [4] */
		linelog_inst ret = { .opcode = JGE, .offset = a1addr + 1 };
		appendinst(ret);
	}
	#undef appendinst
	linelog_inst jge0 = { .opcode = JGE, .rev = 0, .offset = oldlen };
	mustsuccess(writeinst(buf, &jge0, a1addr)); /* [5] */

	/* step IV: write back updated inst0 */
	if (brev > inst0.rev)
		inst0.rev = brev;
	mustsuccess(writeinst(buf, &inst0, 0));

	/* step V: update annotateresult */
	ar->lines[a1].offset = a1newaddr; /* a1inst got moved */
	if (a2 - a1 != b2 - b1) {
		size_t movesize = sizeof(linelog_lineinfo) *
			(ar->linecount + 1 - a2);
		/* the memmove is safe as step II reserved the memory */
		memmove(ar->lines + a1 + b2 - b1, ar->lines + a2, movesize);
		ar->linecount = (linelog_linenum)newlinecount;
	}
	for (linelog_linenum i = b1; i < b2; ++i) {
		linelog_lineinfo *li = ar->lines + a1 + i - b1;
		li->rev = brevs ? brevs[i] : brev;
		li->linenum = blinenums ? blinenums[i] : i;
		li->offset = oldlen + i - b1 + 1;
	}

	return LINELOG_RESULT_OK;
}

linelog_result linelog_replacelines(linelog_buf *buf,
		linelog_annotateresult *ar, linelog_revnum brev,
		linelog_linenum a1, linelog_linenum a2,
		linelog_linenum b1, linelog_linenum b2) {
	return replacelines(buf, ar, brev, a1, a2, b1, b2, NULL, NULL);
}

linelog_result linelog_replacelines_vec(linelog_buf *buf,
		linelog_annotateresult *ar, linelog_revnum brev,
		linelog_linenum a1, linelog_linenum a2,
		linelog_linenum blinecount, const linelog_revnum *brevs,
		const linelog_linenum *blinenums) {
	return replacelines(buf, ar, brev, a1, a2, 0, blinecount,
			brevs, blinenums);
}

linelog_result linelog_getalllines(linelog_buf *buf,
		linelog_annotateresult *ar, linelog_offset offset1,
		linelog_offset offset2) {
	linelog_inst inst0;
	returnonerror(readinst(buf, &inst0, 0));

	linelog_offset pc, nextpc = offset1 ? offset1 : 1;
	ar->linecount = 0;

	for (linelog_offset step = inst0.offset; step; --step) {
		pc = nextpc++;
		if (pc == offset2 || pc == 0)
			return LINELOG_RESULT_OK;

		linelog_inst i;
		returnonerror(readinst(buf, &i, pc));

		switch (i.opcode) {
		case JGE:
			if (i.rev == 0)  /* unconditional jump */
				nextpc = i.offset;
			break;
		case JL:
			break;
		case LINE: /* append a line */
			returnonerror(appendline(ar, &i, pc));
			break;
		default: /* unknown opcode */
			return LINELOG_RESULT_EILLDATA;
		}
	}

	/* step reaches 0, didn't meet the end condition */
	return LINELOG_RESULT_EILLDATA;
}