commit a92e39dbecd8ecc08e77d68d2bbf7153f97fdd34
Author: Bruno BELANYI <bruno@belanyi.fr>
Date:   Wed Oct 28 15:30:54 2020 +0100

    evalexpr: initial recursive descent parser

diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..8dfccac
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,23 @@
+CC = gcc
+CPPFLAGS = -Isrc/ -D_POSIX_C_SOURCE=200809L
+CFLAGS = -Wall -Wextra -pedantic -Werror -std=c99
+VPATH = src/
+
+SRC = \
+    src/ast/ast.c \
+    src/eval/eval.c \
+    src/parse/parse.c \
+
+BIN = evalexpr
+OBJ = $(SRC:.c=.o)
+
+.PHONY: all
+all: $(BIN)
+
+# Write this one rule instead of using the implicit rules to buid at the root
+$(BIN): $(OBJ) src/evalexpr.o
+
+.PHONY: clean
+clean:
+	$(RM) $(OBJ) # remove object files
+	$(RM) $(BIN) # remove main program
diff --git a/src/ast/ast.c b/src/ast/ast.c
new file mode 100644
index 0000000..4db6bb8
--- /dev/null
+++ b/src/ast/ast.c
@@ -0,0 +1,67 @@
+#include "ast.h"
+
+#include <stdlib.h>
+
+struct ast_node *make_num(int val)
+{
+    struct ast_node *ret = malloc(sizeof(*ret));
+
+    if (ret == NULL)
+        return ret;
+
+    ret->kind = NODE_NUM;
+    ret->val.num = val;
+
+    return ret;
+}
+
+struct ast_node *make_unop(enum unop_kind op, struct ast_node *tree)
+{
+    struct ast_node *ret = malloc(sizeof(*ret));
+
+    if (ret == NULL)
+        return ret;
+
+    ret->kind = NODE_UNOP;
+    ret->val.un_op.op = op;
+    ret->val.un_op.tree = tree;
+
+    return ret;
+}
+
+struct ast_node *make_binop(enum binop_kind op, struct ast_node *lhs,
+                            struct ast_node *rhs)
+{
+    struct ast_node *ret = malloc(sizeof(*ret));
+
+    if (ret == NULL)
+        return ret;
+
+    ret->kind = NODE_BINOP;
+    ret->val.bin_op.op = op;
+    ret->val.bin_op.lhs = lhs;
+    ret->val.bin_op.rhs = rhs;
+
+    return ret;
+}
+
+void destroy_ast(struct ast_node *ast)
+{
+    if (!ast)
+        return;
+
+    switch (ast->kind)
+    {
+    case NODE_BINOP:
+        destroy_ast(ast->val.bin_op.lhs);
+        destroy_ast(ast->val.bin_op.rhs);
+        break;
+    case NODE_UNOP:
+        destroy_ast(ast->val.un_op.tree);
+        /* fallthrough */
+    case NODE_NUM:
+        break;
+    }
+
+    free(ast);
+}
diff --git a/src/ast/ast.h b/src/ast/ast.h
new file mode 100644
index 0000000..d992e62
--- /dev/null
+++ b/src/ast/ast.h
@@ -0,0 +1,61 @@
+#ifndef AST_H
+#define AST_H
+
+// Forward declaration
+struct ast_node;
+
+enum unop_kind
+{
+    UNOP_IDENTITY,
+    UNOP_NEGATE,
+    UNOP_FACT,
+};
+
+enum binop_kind
+{
+    BINOP_PLUS,
+    BINOP_MINUS,
+    BINOP_TIMES,
+    BINOP_DIVIDES,
+    BINOP_POW,
+};
+
+struct unop_node
+{
+    enum unop_kind op;
+    struct ast_node *tree;
+};
+
+struct binop_node
+{
+    enum binop_kind op;
+    struct ast_node *lhs;
+    struct ast_node *rhs;
+};
+
+struct ast_node
+{
+    enum node_kind
+    {
+        NODE_UNOP,
+        NODE_BINOP,
+        NODE_NUM,
+    } kind;
+    union ast_val
+    {
+        struct unop_node un_op;
+        struct binop_node bin_op;
+        int num;
+    } val;
+};
+
+struct ast_node *make_num(int val);
+
+struct ast_node *make_unop(enum unop_kind op, struct ast_node *tree);
+
+struct ast_node *make_binop(enum binop_kind op, struct ast_node *lhs,
+                            struct ast_node *rhs);
+
+void destroy_ast(struct ast_node *ast);
+
+#endif /* !AST_H */
diff --git a/src/eval/eval.c b/src/eval/eval.c
new file mode 100644
index 0000000..facb31a
--- /dev/null
+++ b/src/eval/eval.c
@@ -0,0 +1,69 @@
+#include "eval.h"
+
+#define UNREACHABLE() __builtin_unreachable()
+
+static int my_pow(int lhs, int rhs)
+{
+    if (!rhs)
+        return 1;
+    int rec = my_pow(lhs * lhs, rhs / 2);
+    if (rhs & 1)
+        rec *= lhs;
+    return rec;
+}
+
+static int my_fact(int num)
+{
+    int ret = 1;
+    while (num > 1)
+        ret *= num--;
+    return ret;
+}
+
+static int eval_unop(const struct unop_node *un_op)
+{
+    switch (un_op->op)
+    {
+    case UNOP_IDENTITY:
+        return eval_ast(un_op->tree);
+    case UNOP_NEGATE:
+        return -eval_ast(un_op->tree);
+    case UNOP_FACT:
+        return my_fact(eval_ast(un_op->tree));
+    }
+    UNREACHABLE();
+}
+
+static int eval_binop(const struct binop_node *bin_op)
+{
+#define EVAL_OP(OP, TREE) (eval_ast((TREE)->lhs) OP eval_ast((TREE)->rhs))
+    switch (bin_op->op)
+    {
+    case BINOP_PLUS:
+        return EVAL_OP(+, bin_op);
+    case BINOP_MINUS:
+        return EVAL_OP(-, bin_op);
+    case BINOP_TIMES:
+        return EVAL_OP(*, bin_op);
+    case BINOP_DIVIDES:
+        return EVAL_OP(/, bin_op);
+    case BINOP_POW:
+        return my_pow(eval_ast(bin_op->lhs), eval_ast(bin_op->rhs));
+    }
+#undef EVAL_OP
+    UNREACHABLE();
+}
+
+int eval_ast(const struct ast_node *ast)
+{
+    switch (ast->kind)
+    {
+    case NODE_NUM:
+        return ast->val.num;
+    case NODE_UNOP:
+        return eval_unop(&ast->val.un_op);
+    case NODE_BINOP:
+        return eval_binop(&ast->val.bin_op);
+    }
+    UNREACHABLE();
+}
diff --git a/src/eval/eval.h b/src/eval/eval.h
new file mode 100644
index 0000000..49d0a89
--- /dev/null
+++ b/src/eval/eval.h
@@ -0,0 +1,8 @@
+#ifndef EVAL_H
+#define EVAL_H
+
+#include "ast/ast.h"
+
+int eval_ast(const struct ast_node *ast);
+
+#endif /* !EVAL_H */
diff --git a/src/evalexpr.c b/src/evalexpr.c
new file mode 100644
index 0000000..15f1d1c
--- /dev/null
+++ b/src/evalexpr.c
@@ -0,0 +1,33 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+#include "ast/ast.h"
+#include "eval/eval.h"
+#include "parse/parse.h"
+
+int main(void)
+{
+    char *line = NULL;
+    size_t size = 0;
+    ssize_t ret = 0;
+
+    while ((getline(&line, &size, stdin)) > 0)
+    {
+        struct ast_node *ast = parse_string(line);
+
+        if (ast == NULL)
+        {
+            fputs("Could not parse input\n", stderr);
+            ret = 1;
+            continue;
+        }
+
+        printf("%d\n", eval_ast(ast));
+
+        destroy_ast(ast);
+    }
+
+    free(line);
+
+    return ret;
+}
diff --git a/src/parse/parse.c b/src/parse/parse.c
new file mode 100644
index 0000000..a644488
--- /dev/null
+++ b/src/parse/parse.c
@@ -0,0 +1,267 @@
+#include "parse.h"
+
+#include <ctype.h>
+#include <stdbool.h>
+#include <stddef.h>
+
+#include "ast/ast.h"
+
+#define UNREACHABLE() __builtin_unreachable()
+
+static struct ast_node *parse_expression(const char **input);
+static struct ast_node *parse_term(const char **input);
+static struct ast_node *parse_factor(const char **input);
+static struct ast_node *parse_power(const char **input);
+static struct ast_node *parse_group(const char **input);
+
+static void eat_char(const char **input)
+{
+    *input += 1; // Skip this character
+}
+
+static void skip_whitespace(const char **input)
+{
+    while (*input[0] && isspace(*input[0]))
+        eat_char(input);
+}
+
+/*
+** Simple recursive descent using the following grammar, using E as start:
+**
+**      E : T [ ('+'|'-') T ]*
+**      T : F [ ('*'|'/') F ]*
+**      F : [ ('-'|'+') ]* P
+**      P : G [ ('^') F ]*
+**      G : '(' E ')' | CONSTANT [ '!' ]
+**
+** Whitespace is ignored in the input string, only serving to delimit numbers.
+**
+** The input shall consist of a single expression, having a trailing
+** expression in the input results in an error.
+**/
+
+struct ast_node *parse_string(const char *input)
+{
+    if (input == NULL)
+        return NULL;
+
+    struct ast_node *ast = parse_expression(&input);
+
+    if (ast == NULL)
+        return NULL;
+
+    // Make sure there is no trailing character, except whitespace
+    skip_whitespace(&input);
+    if (input[0] != '\0')
+    {
+        destroy_ast(ast);
+        return NULL;
+    }
+
+    return ast;
+}
+
+static enum binop_kind char_to_binop(char c)
+{
+    switch (c)
+    {
+    case '+':
+        return BINOP_PLUS;
+    case '-':
+        return BINOP_MINUS;
+    case '*':
+        return BINOP_TIMES;
+    case '/':
+        return BINOP_DIVIDES;
+    case '^':
+        return BINOP_POW;
+    }
+    UNREACHABLE();
+}
+
+static struct ast_node *parse_expression(const char **input)
+{
+    struct ast_node *lhs = parse_term(input);
+    if (lhs == NULL) // Error occured, abort
+        return NULL;
+
+    do
+    {
+        skip_whitespace(input); // Whitespace is not significant
+
+        if (*input[0] == '\0') // End of input, return parsed expression
+            return lhs;
+
+        if (*input[0] == '+' || *input[0] == '-')
+        {
+            const enum binop_kind op = char_to_binop(*input[0]);
+
+            eat_char(input);
+
+            struct ast_node *rhs = parse_term(input);
+
+            if (rhs == NULL) // Error occured
+            {
+                destroy_ast(lhs);
+                return NULL;
+            }
+
+            lhs = make_binop(op, lhs, rhs);
+        }
+        else
+            break; // Unexpected character, end of loop
+    } while (true);
+
+    return lhs;
+}
+
+static struct ast_node *parse_term(const char **input)
+{
+    struct ast_node *lhs = parse_factor(input);
+    if (lhs == NULL) // Error occured, abort
+        return NULL;
+
+    do
+    {
+        skip_whitespace(input); // Whitespace is not significant
+
+        if (*input[0] == '\0') // End of input, return parsed expression
+            return lhs;
+
+        if (*input[0] == '*' || *input[0] == '/')
+        {
+            const enum binop_kind op = char_to_binop(*input[0]);
+
+            eat_char(input);
+
+            struct ast_node *rhs = parse_factor(input);
+
+            if (rhs == NULL) // Error occured
+            {
+                destroy_ast(lhs);
+                return NULL;
+            }
+
+            lhs = make_binop(op, lhs, rhs);
+        }
+        else
+            break; // Unexpected character, end of loop
+    } while (true);
+
+    return lhs;
+}
+
+static enum unop_kind char_to_unop(char c)
+{
+    switch (c)
+    {
+    case '+': // Assume non-faulty input, return identity as default
+        return UNOP_IDENTITY;
+    case '-':
+        return UNOP_NEGATE;
+    case '!':
+        return UNOP_FACT;
+    }
+    UNREACHABLE();
+}
+
+static struct ast_node *parse_factor(const char **input)
+{
+    skip_whitespace(input); // Whitespace is not significant
+    while (*input[0] == '+' || *input[0] == '-')
+    {
+        const enum unop_kind op = char_to_unop(*input[0]);
+
+        eat_char(input);
+
+        struct ast_node *rhs = parse_factor(input); // Loop by recursion
+
+        if (rhs == NULL)
+            return NULL;
+
+        return make_unop(op, rhs);
+    }
+    return parse_power(input);
+}
+
+static struct ast_node *parse_power(const char **input)
+{
+    struct ast_node *lhs = parse_group(input);
+    if (lhs == NULL) // Error occured, abort
+        return NULL;
+
+    skip_whitespace(input); // Whitespace is not significant
+
+    if (*input[0] == '\0') // End of input, return parsed expression
+        return lhs;
+
+    if (*input[0] == '^')
+    {
+        const enum binop_kind op = char_to_binop(*input[0]);
+
+        eat_char(input);
+
+        struct ast_node *rhs = parse_factor(input);
+
+        if (rhs == NULL) // Error occured
+        {
+            destroy_ast(lhs);
+            return NULL;
+        }
+
+        lhs = make_binop(op, lhs, rhs);
+    }
+
+    return lhs;
+}
+
+static bool my_atoi(const char **input, int *val)
+{
+    if (!isdigit(*input[0]))
+        return false;
+
+    *val = 0; // Initialize its value
+    do
+    {
+        *val *= 10;
+        *val += *input[0] - '0';
+        *input += 1;
+    } while (isdigit(*input[0]));
+
+    return true;
+}
+
+static struct ast_node *parse_group(const char **input)
+{
+    skip_whitespace(input); // Whitespace is not significant
+    struct ast_node *ast = NULL;
+
+    int val = 0;
+    if (my_atoi(input, &val))
+        ast = make_num(val);
+    else if (*input[0] == '(')
+    {
+        // Remove the parenthesis
+        eat_char(input);
+        ast = parse_expression(input);
+        // Check that we have our closing parenthesis
+        skip_whitespace(input);
+        if (*input[0] != ')')
+        {
+            destroy_ast(ast);
+            return NULL;
+        }
+        // Remove the parenthesis
+        eat_char(input);
+        return ast;
+    }
+
+    skip_whitespace(input);
+    if (*input[0] == '!')
+    {
+        eat_char(input);
+        return make_unop(UNOP_FACT, ast);
+    }
+
+    return ast;
+}
diff --git a/src/parse/parse.h b/src/parse/parse.h
new file mode 100644
index 0000000..0121bc7
--- /dev/null
+++ b/src/parse/parse.h
@@ -0,0 +1,8 @@
+#ifndef PARSE_H
+#define PARSE_H
+
+#include "ast/ast.h"
+
+struct ast_node *parse_string(const char *input);
+
+#endif /* !PARSE_H */