db.c

#define  _GNU_SOURCE
#include <stdio.h>
#include <string.h>
#include <stdbool.h>
#include <stdlib.h>
#include <sys/types.h>
#include <stdint.h>

#define COLUMN_USERNAME_SIZE 32
#define COLUMN_EMAIL_SIZE 255 
#define size_of_attribute(Struct, Attribute) sizeof(((Struct*)0)->Attribute)

#define TABLE_MAX_PAGES 100

/**
 * @link https://jameshfisher.com/2017/02/22/ssize_t
 * ssize_t is the same as size_t, but is a signed type - read ssize_t as “signed size_t”. 
 * ssize_t is able to represent the number -1, which is returned by several system calls and 
 * library functions as a way to indicate error. For example, the read and write system calls:
 */
typedef struct {
    char* buffer;
    size_t buffer_length;
    ssize_t input_length;
} InputBuffer;

typedef enum {
    META_COMMAND_SUCCESS,
    META_COMMAND_UNRECOGNIZED_COMMAND
} MetaCommandResult;

typedef enum {
    PREPARE_SUCCESS,
    PREPARE_SYNTAX_ERROR,
    PREPARE_UNRECOGNIZED_STATEMENT
} PrepareResult;

typedef enum {
    STATEMENT_INSERT,
    STATEMENT_SELECT
} StatementType;

typedef enum {
    EXECUTE_TABLE_FULL,
    EXECUTE_SUCCESS
} ExecuteResult;

typedef struct {
    u_int32_t id;
    char username[COLUMN_USERNAME_SIZE];
    char email[COLUMN_EMAIL_SIZE];
} Row;

typedef struct {
    StatementType type;
    Row row_to_insert;
} Statement;

const u_int32_t ID_SIZE = size_of_attribute(Row, id);
const u_int32_t USERNAME_SIZE = size_of_attribute(Row, username);
const u_int32_t EMAIL_SIZE = size_of_attribute(Row, email);

const u_int32_t ID_OFFSET = 0;
const u_int32_t USERNAME_OFFSET = ID_OFFSET + ID_SIZE;
const u_int32_t EMAIL_OFFSET = USERNAME_OFFSET + USERNAME_SIZE;
const u_int32_t ROW_SIZE = ID_SIZE + USERNAME_SIZE + EMAIL_OFFSET;

const u_int32_t PAGE_SIZE = 4096;
const u_int32_t ROWS_PER_PAGE = PAGE_SIZE / ROW_SIZE;
const u_int32_t TABLE_MAX_ROWS = ROWS_PER_PAGE * TABLE_MAX_PAGES;

typedef struct {
    u_int32_t num_rows;
    void* pages[TABLE_MAX_PAGES];
} Table;

InputBuffer* new_input_buffer() {
    InputBuffer* input_buffer = (InputBuffer*)malloc(sizeof(InputBuffer));
    input_buffer->buffer = NULL;
    input_buffer->buffer_length = 0;
    input_buffer->input_length = 0;

    return input_buffer;
}

void print_prompt() {
    printf("db > ");
}

void read_input(InputBuffer* input_buffer) {
    ssize_t bytes_read = 
        getline(&(input_buffer->buffer), &(input_buffer->buffer_length), stdin);

    if (bytes_read <= 0) {
        printf("Error reading input\n");
        exit(EXIT_FAILURE);
    }

    input_buffer->input_length = bytes_read - 1;
    input_buffer->buffer[bytes_read - 1] = 0;
}

void close_input_buffer(InputBuffer* input_buffer) {
    free(input_buffer->buffer);
    free(input_buffer);
}

MetaCommandResult do_meta_command(InputBuffer* input_buffer) {
    if (strcmp(input_buffer->buffer, ".exit") == 0) {
        exit(EXIT_SUCCESS);
    } else {
        return META_COMMAND_UNRECOGNIZED_COMMAND;
    }
}

PrepareResult prepare_statement(InputBuffer* input_buffer, Statement* statement) {
    if (strncmp(input_buffer->buffer, "insert", 6) == 0) {
        statement->type = STATEMENT_INSERT;
        int args_assigned = sscanf(input_buffer->buffer, "insert %u %s %s", &(statement->row_to_insert.id),
            statement->row_to_insert.username, statement->row_to_insert.email);
        if (args_assigned < 3) {
            return PREPARE_SYNTAX_ERROR;
        }
        
        return PREPARE_SUCCESS;
    }
    
    if (strncmp(input_buffer->buffer, "select", 6) == 0) {
        statement->type = STATEMENT_SELECT;
        return PREPARE_SUCCESS;
    }

    return PREPARE_UNRECOGNIZED_STATEMENT;
}

void print_row(Row* row) {
    printf("(%d, %s, %s)\n", row->id, row->username, row->email);
}

void serialize_row(Row* source, void* destination) {
    memcpy(destination + ID_OFFSET, &(source->id), ID_SIZE);
    memcpy(destination + USERNAME_OFFSET, &(source->username), USERNAME_SIZE);
    memcpy(destination + EMAIL_OFFSET, &(source->email), EMAIL_SIZE);
}

void deserialize_row(void* source, Row* destination) {
    memcpy(&(destination->id), source + ID_OFFSET, ID_SIZE);
    memcpy(&(destination->username), source + USERNAME_OFFSET, USERNAME_SIZE);
    memcpy(&(destination->email), source + EMAIL_OFFSET, EMAIL_SIZE);
}

void* row_slot(Table* table, u_int32_t row_num) {
    u_int32_t page_num = row_num / ROWS_PER_PAGE;
    void* page = table->pages[page_num];
    if (page == NULL) {
        page = table->pages[page_num] = malloc(PAGE_SIZE);
    }
    u_int32_t row_offset = row_num % ROWS_PER_PAGE;
    u_int32_t byte_offset = row_offset * ROW_SIZE;
    
    return page + byte_offset;
}

ExecuteResult execute_insert(Statement* statement, Table* table) {
    if (table->num_rows >= TABLE_MAX_ROWS) {
        return EXECUTE_TABLE_FULL;
    }

    Row* row_to_insert = &(statement->row_to_insert);

    serialize_row(row_to_insert, row_slot(table, table->num_rows));
    table->num_rows += 1;

    return EXECUTE_SUCCESS;
}

ExecuteResult execute_select(Statement* statement, Table* table) {
    Row row;
    for (u_int32_t i = 0; i < table->num_rows; i++) {
        deserialize_row(row_slot(table, i), &row);
        print_row(&row);
    }

    return EXECUTE_SUCCESS;
}

ExecuteResult execute_statement(Statement* statement, Table* table) {
    switch (statement->type) {
        case STATEMENT_INSERT:
            return execute_insert(statement, table);
        case STATEMENT_SELECT:
            return execute_select(statement, table);
    }
}

Table* new_table() {
    Table* table = (Table*)malloc(sizeof(Table));
    table->num_rows = 0;
    for (u_int32_t i = 0; i < TABLE_MAX_PAGES; i++) {
        table->pages[i] = NULL;
    }
    return table;
}

void free_table(Table* table) {
    for (int i = 0; table->pages[i]; i++) {
        free(table->pages[i]);
    }
    free(table);
}

int main() {
    Table* table = new_table();
    InputBuffer* input_buffer = new_input_buffer();
    while(true) {
        print_prompt();
        read_input(input_buffer);

        if (input_buffer->buffer[0] == '.') {
            switch (do_meta_command(input_buffer)) {
                case META_COMMAND_SUCCESS:
                    continue;
                case META_COMMAND_UNRECOGNIZED_COMMAND:
                    printf("Unrecognized command '%s'\n", input_buffer->buffer);
                    continue;
            }
        }

        Statement* statement = (Statement*)malloc(sizeof(Statement));
        switch (prepare_statement(input_buffer, statement)) {
            case PREPARE_SUCCESS:
                break;
            case PREPARE_SYNTAX_ERROR:
                printf("Syntax error. Could not parse statement\n");
                continue;
            case PREPARE_UNRECOGNIZED_STATEMENT:
                printf("Unrecognized keyword at start of '%s'\n", input_buffer->buffer);
                continue;
        }

        switch(execute_statement(statement, table)) {
            case (EXECUTE_SUCCESS):
                printf("Executed.\n");
                break;
            case (EXECUTE_TABLE_FULL):
                printf("Error: Table full.\n");
                break;
        }
    }
}