/*
 * Copyright (c) 2024 DTB <trinity@trinity.moe>
 * SPDX-License-Identifier: AGPL-3.0-or-later
 *
 * This program is free software: you can redistribute it and/or modify it under
 * the terms of the GNU Affero General Public License as published by the Free
 * Software Foundation, either version 3 of the License, or (at your option) any
 * later version.
 *
 * This program is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
 * FOR A PARTICULAR PURPOSE. See the GNU Affero General Public License for more
 * details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program. If not, see https://www.gnu.org/licenses/.
 */

#include <errno.h> /* errno */
#include <stdio.h> /* fclose(3), feof(3), fgetc(3), fgets(3), fopen(3),
                    * fprintf(3), fputc(3), stderr, stdin, stdout, EOF, FILE,
                    * NULL */
#include <stdlib.h> /* size_t */
#include <string.h> /* strchr(3), strcmp(3), strerror(3), strtok(3) */
#include <unistd.h> /* getopt(3) */

/* Commands start with cmd_. They take an argc and NULL-terminated argv, like
 * main, and return a status from <sysexits.h>. Return values other than EX_OK
 * and EX_USAGE cause pg(1) to exit with that value, except EX_UNAVAILABLE,
 * which causes pg(1) to exit with the status EX_OK. */
#include <sysexits.h>

#define CMDLINE_MAX 99+1 /* Maximum length of command line. */

static char *whitespace = " \n\r\t\v";

static struct {
	size_t quantity;
	enum { LINES = 0, BYTES = 1 } type;
} default_page_unit = { 22, LINES } /* Plan 9 default */;
static FILE *input;
static char *prompt = ": ";
static char *program_name = "pg";

static char *permute_out(char *str, size_t index){
	size_t j;

	for(j = index; str[j - 1] != '\0'; ++j)
		str[j - 1] = str[j];

	return str;
}

/* strtok(3p), but supports double-quotes and escapes (but only for escaping
 * quotes). UTF-8 is safe only in str. Unmatched quotes in str are considered
 * literal. The behavior of strtok_quoted when '"' or '\\' are in sep is
 * undefined. */
/* TODO: Seems to only ever return NULL. */
static char *strtok_quoted(char *str, char *sep){
	static char *s;
	size_t i;
	size_t j;

	if(str != NULL)
		s = str;

	while(strchr(sep, *s) == NULL)
		if(*++s == '\0')
			return NULL; /* no remaining characters except seperators */

	{
		char in_escape;
		int in_quotes;

		in_escape = 0;
		in_quotes = -1;
		for(i = 0; s[i] != '\0'; ++i)
			switch(s[i]){
			case '\\':
				if(in_escape)
					permute_out(s, i--);
				in_escape = !in_escape;
				break;
			case '"':
				if(in_escape){
					s[i] = s[i - 1];
					permute_out(s, i--);
				}else if(in_quotes != -1){
					permute_out(s, in_quotes);
					--i;
					permute_out(s, i--);
					in_quotes = -1;
				}else
					in_quotes = i;
				break;
			case '\0':
				return s;
			default:
				if(!in_escape && strchr(sep, s[i]) != NULL){
					s[i] = '\0';
					return s;
				}
			}
	}

	return s;
}

/* Page at most l bytes from f without consideration of a buffer (print them to
 * stdout). */
static int pg_b_u(FILE *f, size_t l){
	int c;

	while((c = fgetc(f)) != EOF)
		if((c = fputc(c, stdout)) == EOF || --l == 0)
			break;

	return c;
}

/* Page at most l lines, which are lengths of characters terminated by nl, from
 * f without consideration of a buffer (print them to stdout). */
static int pg_l_u(FILE *f, size_t l, char nl){
	int c;

	while((c = fgetc(f)) != EOF)
		if((c = fputc(c, stdout)) == EOF || (l -= (c == nl)) == 0)
			break;

	return c;
}

static int cmd_quit(int argc, char **argv, char **envp){return EX_UNAVAILABLE;}
static int cmd_default_page(int argc, char **argv, char **envp){

	if(argc > 1) /* This shouldn't be possible. */
		return EX_USAGE;
	if(default_page_unit.type == BYTES)
		return pg_b_u(input, default_page_unit.quantity) == EOF
			? EX_UNAVAILABLE
			: EX_OK;
	else if(default_page_unit.type == LINES)
		return pg_l_u(input, default_page_unit.quantity, '\n') == EOF
			? EX_UNAVAILABLE
			: EX_OK;
	else
		return EX_SOFTWARE;
}
static int cmd_page_down_lines(int argc, char **argv, char **envp){

	switch(argc){
	case 1:
		return cmd_default_page(argc, argv, envp);
	case 2: /* not implemented */
	default:
		fprintf(stderr, "Usage: %s" /*" (lines)"*/ "\n", argv[0]);
		return EX_USAGE;
	}
}

/* A CmdMap must be NULL-terminated. */
static struct CmdMap{ char *name; int (*fn)(int, char **, char **); }
builtins[] = {
	{ "", cmd_default_page },
	{ "+", cmd_page_down_lines },

	/* don't make the user feel trapped */
	{ ":q", cmd_quit }, { ":q!", cmd_quit },
	{ "exit", cmd_quit }, { "q", cmd_quit }, { "Q", cmd_quit },
	{ "quit", cmd_quit }, { "ZZ", cmd_quit },
	
	{ NULL, NULL }
};

#define ARGV_MAX 10
/* Find and execute the command in the command map, given a corresponding
 * command line. */
static int cmdline_exec(struct CmdMap *map, char *cmdline, char **envp){
	/* Command line word splitting is naive and based on whitespace ONLY; no
	 * fancy quoting or escaping here. Adding that would (ideally) entail
	 * replacing strtok(3) with something specific to this task. */
	static int argc;
	static char *argv[ARGV_MAX];

	if((argv[(argc = 0)] = strtok(cmdline, whitespace)) == NULL){
		while(cmdline[0] != '\0')
			cmdline = &cmdline[1];
		argv[argc] = cmdline;
		argv[++argc] = NULL;
	}else{
		while((argv[++argc] = strtok(NULL, whitespace)) != NULL
		      && argc < ARGV_MAX);
	}

	for(; map->name != NULL; map = &map[1])
		if(strcmp(map->name, argv[0]) == 0)
			return map->fn(argc, argv, envp);

	fprintf(stderr, "%s: %s: not found\n", program_name, argv[0]);
	return EX_USAGE;
}

int usage(char *s){
	fprintf(stderr, "Usage: %s (-p [prompt])\n", s);
	return EX_USAGE;
}

int main(int argc, char *argv[]){
	unsigned char cmd[CMDLINE_MAX];
	FILE *t;

	input = stdin;

	if(argc < 1)
		program_name = argv[0];
	else{
		int c;

		while((c = getopt(argc, argv, "p:")) != -1)
			switch(c){
			case 'p': prompt = optarg; break;
			default:
				return usage(program_name);
			}
	}

	if(argc > optind)
		return usage(program_name);


	if((t = fopen("/dev/tty", "rb")) == NULL){
		fprintf(stderr, "%s: /dev/tty: %s\n", program_name, strerror(errno));
		return EX_OSERR;
	}

	for(;;){
		fputs(prompt, stderr);
		fgets((char *)cmd, (sizeof cmd) / (sizeof *cmd), t);
		if(strchr((char *)cmd, '\n') == NULL){ /* fast-forward stream */
			int c;

			while((c = fgetc(t)) != '\n')
				if(c == EOF)
					break;
		}

		if(feof(t))
			return EX_OK;

		{	int r;
			switch((r = cmdline_exec(builtins, (char *)cmd, NULL))){
			case EX_OK: case EX_USAGE: break;
			case EX_UNAVAILABLE: return EX_OK;
			default: return r;
		}	}
	}

	/* UNREACHABLE */
	return EX_SOFTWARE;
}