sqlite.c

#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <sys/types.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
// #include <teste.h>

#define COLUMN_USERNAME_SIZE 64
#define COLUMN_EMAIL_SIZE 255

#define size_of_attribute(Struct, Attribute) sizeof(((Struct*)0)->Attribute)

#define TABLE_MAX_PAGES 100

typedef struct {
    char* buffer;
    size_t buffer_length;
    ssize_t input_length;
} InputBuffer;

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 > "); }

ssize_t getline(char **lineptr, size_t *n, FILE *stream);

// lineptr : a pointer to the variable we use to point to the buffer containing the read line.
// If it set to NULL it is mallocatted by getline and should thus be freed by the user, even if the command fails.
// return value : the number of bytes read, which may be less than the size of the buffer.

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);
    }

    // Ignore trailing newline
    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);
}

typedef enum {
    EXECUTE_SUCCESS,
    EXECUTE_TABLE_FULL
} ExecuteResult;

typedef enum {
  META_COMMAND_SUCCESS,
  META_COMMAND_UNRECOGNIZED_COMMAND
} MetaCommandResult;

typedef enum {
    PREPARE_SUCCESS,
    PREPARE_SYNTAX_ERROR,
    PREPARE_UNRECOGNIZED_STATEMENT,
    PREPARE_STRING_TOO_LONG,
    PREPARE_NEGATIVE_ID,
} PrepareResult;

// Our “prepared statement” right now just contains an enum with two possible values. It will contain more data as we allow parameters in statements:
typedef enum {
    STATEMENT_INSERT,
    STATEMENT_SELECT
} StatementType;

typedef struct {
    uint32_t id;
    char username[COLUMN_USERNAME_SIZE + 1];
    char email[COLUMN_EMAIL_SIZE + 1];
} Row;

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

typedef struct {
    StatementType type;
    Row row_to_insert;  // only used by insert statement
} Statement;

const uint32_t ID_SIZE = size_of_attribute(Row, id);
const uint32_t USERNAME_SIZE = size_of_attribute(Row, username);
const uint32_t EMAIL_SIZE = size_of_attribute(Row, email);
const uint32_t ID_OFFSET = 0;
const uint32_t USERNAME_OFFSET = ID_OFFSET + ID_SIZE;
const uint32_t EMAIL_OFFSET = USERNAME_OFFSET + USERNAME_SIZE;
const uint32_t ROW_SIZE = ID_SIZE + USERNAME_SIZE + EMAIL_SIZE;

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);
}

// Next, a Table structure that points to pages of rows and keeps track of how many rows there are:
const uint32_t PAGE_SIZE = 4096;
const uint32_t ROWS_PER_PAGE = PAGE_SIZE / ROW_SIZE;
const uint32_t TABLE_MAX_ROWS = ROWS_PER_PAGE * TABLE_MAX_PAGES;

typedef struct {
    int file_descriptor;
    uint32_t file_length;
    void* pages[TABLE_MAX_PAGES];
} Pager;

typedef struct {
  Pager* pager;
  uint32_t num_rows;
} Table;

void* get_page(Pager* pager, uint32_t page_num) {
    if (page_num > TABLE_MAX_PAGES) {
        printf("Tried to fetch page number out of bounds. %d > %d\n", page_num, TABLE_MAX_PAGES);
        exit(EXIT_FAILURE);
    }

    if (pager->pages[page_num] == NULL) {
        // Cache miss. Allocate memory and load from file.
        void* page = malloc(PAGE_SIZE);
        uint32_t num_pages = pager->file_length / PAGE_SIZE;

        // We might save a opartial page at the end of the file
        if (pager->file_length % PAGE_SIZE) {
            num_pages += 1;
        }

        if (page_num <= num_pages) {
            lseek(pager->file_descriptor, page_num * PAGE_SIZE, SEEK_SET);
            ssize_t bytes_read = read(pager->file_descriptor, page, PAGE_SIZE);
            if (bytes_read == -1) {
                printf("Error reading file: %d\n", errno);
                exit(EXIT_FAILURE);
            }
        }

        pager->pages[page_num] = page;
    }
    return pager->pages[page_num];
}

// here is how we figure out where to read/write in memory for a particular row:
void* row_slot(Table* table, uint32_t row_num) {
  uint32_t page_num = row_num / ROWS_PER_PAGE;
  void* page = get_page(table->pager, page_num);
  uint32_t row_offset = row_num % ROWS_PER_PAGE;
  uint32_t byte_offset = row_offset * ROW_SIZE;
  return page + byte_offset;
}

Pager* pager_open(const char* filename) {
    int fd = open(filename,
                    O_RDWR |        // Read/Write mode
                        O_CREAT,    // Create file if it doesn't exist
                    S_IWUSR |       // User write permission
                        S_IRUSR     // User read permission
                    );

    if (fd == -1) {
        printf("Unable to open file\n");
        exit(EXIT_FAILURE);
    }

    off_t file_length = lseek(fd, 0, SEEK_END);

    Pager* pager = malloc(sizeof(Pager));
    pager->file_descriptor = fd;
    pager->file_length = file_length;

    for (uint32_t i = 0; i < TABLE_MAX_PAGES; i++) {
        pager->pages[i] = NULL;
    }

    return pager;
}

// Lastly, we need to initialize the table, create the respective memory release function and handle a few more error cases:
Table* db_open(const char* filename) {
    Pager* pager = pager_open(filename);
    uint32_t num_rows = pager->file_length / ROW_SIZE;

    Table* table = malloc(sizeof(Table));
    table->pager = pager;
    table->num_rows = num_rows;

    return table;
}

void pager_flush(Pager* pager, uint32_t page_num, uint32_t size) {
    if (pager->pages[page_num] == NULL) {
        printf("Tried to flush null page\n");
        exit(EXIT_FAILURE);
    }

    off_t offset = lseek(pager->file_descriptor, page_num * PAGE_SIZE, SEEK_SET);

    if (offset == -1) {
        printf("Error seeking: %d\n", errno);
        exit(EXIT_FAILURE);
    }

    ssize_t bytes_written =
        write(pager->file_descriptor, pager->pages[page_num], size);

    if (bytes_written == -1) {
        printf("Error writing: %d\n", errno);
        exit(EXIT_FAILURE);
    }
}

void db_close(Table* table) {
    Pager* pager = table->pager;
    uint32_t num_full_pages = table->num_rows / ROWS_PER_PAGE;

    for (uint32_t i = 0; i < num_full_pages; i++) {
        if (pager->pages[i] == NULL) {
            continue;
        }
        pager_flush(pager, i, PAGE_SIZE);
        free(pager->pages[i]);
        pager->pages[i] = NULL;
    }

    // There may be a partial page to write to the end of the file
    // This should not be needed after we switch to a B-tree
    uint32_t num_additional_rows = table->num_rows % ROWS_PER_PAGE;
    if (num_additional_rows > 0) {
        uint32_t page_num = num_full_pages;
        if (pager->pages[page_num] != NULL) {
            pager_flush(pager, page_num, num_additional_rows * ROW_SIZE);
            free(pager->pages[page_num]);
            pager->pages[page_num] = NULL;
        }
    }

    int result = close(pager->file_descriptor);
    if (result == -1) {
        printf("Error closing db file.\n");
        exit(EXIT_FAILURE);
    }
    for (uint32_t i = 0; i < TABLE_MAX_PAGES; i++) {
        void* page = pager->pages[i];
        if (page) {
            free(page);
            pager->pages[i] = NULL;
        }
    }
    free(pager);
    free(table);
}

// do_meta_command is just a wrapper for existing functionality that leaves room for more commands:
MetaCommandResult do_meta_command(InputBuffer* input_buffer, Table* table) {
  if (strcmp(input_buffer->buffer, ".exit") == 0) {
    db_close(table);
    exit(EXIT_SUCCESS);
  } else {
    return META_COMMAND_UNRECOGNIZED_COMMAND;
  }
}

PrepareResult prepare_insert(InputBuffer* input_buffer, Statement* statement) {
    statement->type = STATEMENT_INSERT;

    char* keyword = strtok(input_buffer->buffer, " ");
    char* id_string = strtok(NULL, " ");
    char* username = strtok(NULL, " ");
    char* email = strtok(NULL, " ");

    if (id_string == NULL || username == NULL || email == NULL) {
        return PREPARE_SYNTAX_ERROR;
    }

    int id = atoi(id_string);
    if (id < 0) {
        return PREPARE_NEGATIVE_ID;
    }

    if (strlen(username) > COLUMN_USERNAME_SIZE) {
        return PREPARE_STRING_TOO_LONG;
    }
    if (strlen(email) > COLUMN_EMAIL_SIZE) {
        return PREPARE_STRING_TOO_LONG;
    }

    statement->row_to_insert.id = id;
    strcpy(statement->row_to_insert.username, username);
    strcpy(statement->row_to_insert.email, email);

    return PREPARE_SUCCESS;
}

// prepare_statement (our “SQL Compiler”) does not understand SQL right now. In fact, it only understands two words:
// Note that we use strncmp for “insert” since the “insert” keyword will be followed by data. (e.g. insert 1 cstack foo@bar.com)
PrepareResult prepare_statement(InputBuffer* input_buffer, Statement* statement) {
    if (strncmp(input_buffer->buffer, "insert", 6) == 0) {
        return prepare_insert(input_buffer, statement);
    }
    if (strcmp(input_buffer->buffer, "select") == 0) {
        statement->type = STATEMENT_SELECT;
        return PREPARE_SUCCESS;
    }

    return PREPARE_UNRECOGNIZED_STATEMENT;
}

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 (uint32_t i = 0; i < table->num_rows; i++) {
        deserialize_row(row_slot(table, i), &row);
        print_row(&row);
    }

    return EXECUTE_SUCCESS;
}

// Note that we use strncmp for “insert” since the “insert” keyword will be followed by data. (e.g. insert 1 cstack foo@bar.com)
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);
    }
}

int main(int argc, char *argv[])
{
    if (argc < 2) {
        printf("Must supply a database filename.\n");
        exit(EXIT_FAILURE);
    }

    char* filename = argv[1];
    Table* table = db_open(filename);


    InputBuffer *input_buffer = new_input_buffer();
    while (1)
    {
        print_prompt();
        read_input(input_buffer);

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

        Statement statement;
        switch (prepare_statement(input_buffer, &statement))
        {
            case (PREPARE_SUCCESS):
                break;
            case (PREPARE_NEGATIVE_ID):
                printf("ID must be positive.\n");
                continue;
            case (PREPARE_STRING_TOO_LONG):
                printf("String is too long.\n");
                continue;
            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;
        }
    }
}