$ go run *.go
Welcome to gosql.
# CREATE TABLE users (id INT, name TEXT);
ok
# INSERT INTO users VALUES (1, 'Phil');
ok
# SELECT id, name FROM users;
| id | name |
====================
| 1 |  Phil |
ok
# INSERT INTO users VALUES (2, 'Kate');
ok
# SELECT name, id FROM users;
| name | id |
====================
| Phil |  1 |
| Kate |  2 |
ok

package gosql

import (
    "fmt"
    "strings"
)

type location struct {
    line uint
    col  uint
}

type keyword string

const (
    selectKeyword keyword = "select"
    fromKeyword   keyword = "from"
    asKeyword     keyword = "as"
    tableKeyword  keyword = "table"
    createKeyword keyword = "create"
    insertKeyword keyword = "insert"
    intoKeyword   keyword = "into"
    valuesKeyword keyword = "values"
    intKeyword    keyword = "int"
    textKeyword   keyword = "text"
)

type symbol string

const (
    semicolonSymbol  symbol = ";"
    asteriskSymbol   symbol = "*"
    commaSymbol      symbol = ","
    leftparenSymbol  symbol = "("
    rightparenSymbol symbol = ")"
)

type tokenKind uint

const (
    keywordKind tokenKind = iota
    symbolKind
    identifierKind
    stringKind
    numericKind
)

type token struct {
    value string
    kind  tokenKind
    loc   location
}

type cursor struct {
    pointer uint
    loc     location
}

func (t *token) equals(other *token) bool {
    return t.value == other.value && t.kind == other.kind
}

type lexer func(string, cursor) (*token, cursor, bool)

func lex(source string) ([]*token, error) {
    tokens := []*token{}
    cur := cursor{}

lex:
    for cur.pointer < uint(len(source)) {
        lexers := []lexer{lexKeyword, lexSymbol, lexString, lexNumeric, lexIdentifier}
        for _, l := range lexers {
            if token, newCursor, ok := l(source, cur); ok {
                cur = newCursor

                // Omit nil tokens for valid, but empty syntax like newlines
                if token != nil {
                    tokens = append(tokens, token)
                }

                continue lex
            }
        }

        hint := ""
        if len(tokens) > 0 {
            hint = " after " + tokens[len(tokens)-1].value
        }
        return nil, fmt.Errorf("Unable to lex token%s, at %d:%d", hint, cur.loc.line, cur.loc.col)
    }

    return tokens, nil
}

func lexNumeric(source string, ic cursor) (*token, cursor, bool) {
    cur := ic

    periodFound := false
    expMarkerFound := false

    for ; cur.pointer < uint(len(source)); cur.pointer++ {
        c := source[cur.pointer]
        cur.loc.col++

        isDigit := c >= '0' && c <= '9'
        isPeriod := c == '.'
        isExpMarker := c == 'e'

        // Must start with a digit or period
        if cur.pointer == ic.pointer {
            if !isDigit && !isPeriod {
                return nil, ic, false
            }

            periodFound = isPeriod
            continue
        }

        if isPeriod {
            if periodFound {
                return nil, ic, false
            }

            periodFound = true
            continue
        }

        if isExpMarker {
            if expMarkerFound {
                return nil, ic, false
            }

            // No periods allowed after expMarker
            periodFound = true
            expMarkerFound = true

            // expMarker must be followed by digits
            if cur.pointer == uint(len(source)-1) {
                return nil, ic, false
            }

            cNext := source[cur.pointer+1]
            if cNext == '-' || cNext == '+' {
                cur.pointer++
                cur.loc.col++
            }

            continue
        }

        if !isDigit {
            break
        }
    }

    // No characters accumulated
    if cur.pointer == ic.pointer {
        return nil, ic, false
    }

    return &token{
        value: source[ic.pointer:cur.pointer],
        loc:   ic.loc,
        kind:  numericKind,
    }, cur, true
}

func lexCharacterDelimited(source string, ic cursor, delimiter byte) (*token, cursor, bool) {
    cur := ic

    if len(source[cur.pointer:]) == 0 {
        return nil, ic, false
    }

    if source[cur.pointer] != delimiter {
        return nil, ic, false
    }

    cur.loc.col++
    cur.pointer++

    var value []byte
    for ; cur.pointer < uint(len(source)); cur.pointer++ {
        c := source[cur.pointer]

        if c == delimiter {
            // SQL escapes are via double characters, not backslash.
            if cur.pointer+1 >= uint(len(source)) || source[cur.pointer+1] != delimiter {
                return &token{
                    value: string(value),
                    loc:   ic.loc,
                    kind:  stringKind,
                }, cur, true
            } else {
                value = append(value, delimiter)
                cur.pointer++
                cur.loc.col++
            }
        }

        value = append(value, c)
        cur.loc.col++
    }

    return nil, ic, false
}

func lexString(source string, ic cursor) (*token, cursor, bool) {
    return lexCharacterDelimited(source, ic, '\'')
}

func lexSymbol(source string, ic cursor) (*token, cursor, bool) {
    c := source[ic.pointer]
    cur := ic
    // Will get overwritten later if not an ignored syntax
    cur.pointer++
    cur.loc.col++

    switch c {
    // Syntax that should be thrown away
    case '\n':
        cur.loc.line++
        cur.loc.col = 0
        fallthrough
    case '\t':
        fallthrough
    case ' ':
        return nil, cur, true
    }

    // Syntax that should be kept
    symbols := []symbol{
        commaSymbol,
        leftParenSymbol,
        rightParenSymbol,
        semicolonSymbol,
        asteriskSymbol,
    }

    var options []string
    for _, s := range symbols {
        options = append(options, string(s))
    }

    // Use `ic`, not `cur`
    match := longestMatch(source, ic, options)
    // Unknown character
    if match == "" {
        return nil, ic, false
    }

    cur.pointer = ic.pointer + uint(len(match))
    cur.loc.col = ic.loc.col + uint(len(match))

    return &token{
        value: match,
        loc:   ic.loc,
        kind:  symbolKind,
    }, cur, true
}

func lexKeyword(source string, ic cursor) (*token, cursor, bool) {
    cur := ic
    keywords := []keyword{
        selectKeyword,
        insertKeyword,
        valuesKeyword,
        tableKeyword,
        createKeyword,
        whereKeyword,
        fromKeyword,
        intoKeyword,
        textKeyword,
    }

    var options []string
    for _, k := range keywords {
        options = append(options, string(k))
    }

    match := longestMatch(source, ic, options)
    if match == "" {
        return nil, ic, false
    }

    cur.pointer = ic.pointer + uint(len(match))
    cur.loc.col = ic.loc.col + uint(len(match))

    return &token{
        value: match,
        kind:  kind,
        loc:   ic.loc,
    }, cur, true
}

// longestMatch iterates through a source string starting at the given
// cursor to find the longest matching substring among the provided
// options
func longestMatch(source string, ic cursor, options []string) string {
    var value []byte
    var skipList []int
    var match string

    cur := ic

    for cur.pointer < uint(len(source)) {

        value = append(value, strings.ToLower(string(source[cur.pointer]))...)
        cur.pointer++

    match:
        for i, option := range options {
            for _, skip := range skipList {
                if i == skip {
                    continue match
                }
            }

            // Deal with cases like INT vs INTO
            if option == string(value) {
                skipList = append(skipList, i)
                if len(option) > len(match) {
                    match = option
                }

                continue
            }

            sharesPrefix := string(value) == option[:cur.pointer-ic.pointer]
            tooLong := len(value) > len(option)
            if tooLong || !sharesPrefix {
                skipList = append(skipList, i)
            }
        }

        if len(skipList) == len(options) {
            break
        }
    }

    return match
}

func lexIdentifier(source string, ic cursor) (*token, cursor, bool) {
    // Handle separately if is a double-quoted identifier
    if token, newCursor, ok := lexCharacterDelimited(source, ic, '"'); ok {
        return token, newCursor, true
    }

    cur := ic

    c := source[cur.pointer]
    // Other characters count too, big ignoring non-ascii for now
    isAlphabetical := (c >= 'A' && c <= 'Z') || (c >= 'a' && c <= 'z')
    if !isAlphabetical {
        return nil, ic, false
    }
    cur.pointer++
    cur.loc.col++

    value := []byte{c}
    for ; cur.pointer < uint(len(source)); cur.pointer++ {
        c = source[cur.pointer]

        // Other characters count too, big ignoring non-ascii for now
        isAlphabetical := (c >= 'A' && c <= 'Z') || (c >= 'a' && c <= 'z')
        isNumeric := c >= '0' && c <= '9'
        if isAlphabetical || isNumeric || c == '$' || c == '_' {
            value = append(value, c)
            cur.loc.col++
            continue
        }

        break
    }

    if len(value) == 0 {
        return nil, ic, false
    }

    return &token{
        // Unquoted dentifiers are case-insensitive
        value: strings.ToLower(string(value)),
        loc:   ic.loc,
        kind:  identifierKind,
    }, cur, true
}

package main

type Ast struct {
    Statements []*Statement
}

type AstKind uint

const (
    SelectKind AstKind = iota
    CreateTableKind
    InsertKind
)

type Statement struct {
    SelectStatement      *SelectStatement
    CreateTableStatement *CreateTableStatement
    InsertStatement      *InsertStatement
    Kind                 AstKind
}

type InsertStatement struct {
    table  token
    values *[]*expression
}

type expressionKind uint

const (
    literalKind expressionKind = iota
)

type expression struct {
    literal *token
    kind    expressionKind
}

type columnDefinition struct {
    name     token
    datatype token
}

type CreateTableStatement struct {
    name token
    cols *[]*columnDefinition
}

type SelectStatement struct {
    item []*expression
    from token
}

package main

import (
    "errors"
    "fmt"
)

func tokenFromKeyword(k keyword) token {
    return token{
        kind:  keywordKind,
        value: string(k),
    }
}

func tokenFromSymbol(s symbol) token {
    return token{
        kind:  symbolKind,
        value: string(s),
    }
}

func expectToken(tokens []*token, cursor uint, t token) bool {
    if cursor >= uint(len(tokens)) {
        return false
    }

    return t.equals(tokens[cursor])
}

func helpMessage(tokens []*token, cursor uint, msg string) {
    var c *token
    if cursor < uint(len(tokens)) {
        c = tokens[cursor]
    } else {
        c = tokens[cursor-1]
    }

    fmt.Printf("[%d,%d]: %s, got: %s\n", c.loc.line, c.loc.col, msg, c.value)
}

func Parse(source string) (*Ast, error) {
    tokens, err := lex(source)
    if err != nil {
        return nil, err
    }

    a := Ast{}
    cursor := uint(0)
    for cursor < uint(len(tokens)) {
        stmt, newCursor, ok := parseStatement(tokens, cursor, tokenFromSymbol(semicolonSymbol))
        if !ok {
            helpMessage(tokens, cursor, "Expected statement")
            return nil, errors.New("Failed to parse, expected statement")
        }
        cursor = newCursor

        a.Statements = append(a.Statements, stmt)

        atLeastOneSemicolon := false
        for expectToken(tokens, cursor, tokenFromSymbol(semicolonSymbol)) {
            cursor++
            atLeastOneSemicolon = true
        }

        if !atLeastOneSemicolon {
            helpMessage(tokens, cursor, "Expected semi-colon delimiter between statements")
            return nil, errors.New("Missing semi-colon between statements")
        }
    }

    return &a, nil
}

func parseStatement(tokens []*token, initialCursor uint, delimiter token) (*Statement, uint, bool) {
    cursor := initialCursor

    // Look for a SELECT statement
    semicolonToken := tokenFromSymbol(semicolonSymbol)
    slct, newCursor, ok := parseSelectStatement(tokens, cursor, semicolonToken)
    if ok {
        return &Statement{
            Kind:            SelectKind,
            SelectStatement: slct,
        }, newCursor, true
    }

    // Look for a INSERT statement
    inst, newCursor, ok := parseInsertStatement(tokens, cursor, semicolonToken)
    if ok {
        return &Statement{
            Kind:            InsertKind,
            InsertStatement: inst,
        }, newCursor, true
    }

    // Look for a CREATE statement
    crtTbl, newCursor, ok := parseCreateTableStatement(tokens, cursor, semicolonToken)
    if ok {
        return &Statement{