# Weekend Project
## Go by Example
### Image Downloader
* [Go by example](https://golangbyexample.com/download-image-file-url-golang/)
* dlv debug with command line argument
* `dlv debug -- https://yourbasic.org/golang/directions.jpg -o logo.png`
* Use `--` to separate (flag) args to dlv itself from args to the debugged binary.

### URL Parsing
* [Go by example](https://gobyexample.com/url-parsing)
>`sandbox/go/weekend_project/url-parsing/`
* :question: What section does URL contain?
* [IBM - The components of a URL](https://www.ibm.com/support/knowledgecenter/SSGMCP_5.2.0/com.ibm.cics.ts.internet.doc/topics/dfhtl_uricomp.html)
* `scheme://host:port/path?query`
* scheme:
* e.g.
* http (without ssl)
* https (with ssl)
* host
* e.g.
* www.google.com
* port
* e.g.
* `80`
* file path
* `/software/htp/cics/index.html`
* query string
* seperate by `&`
* e.g.
* `term=bluebird&source=browser-search`
### Go's URL structure:
* [go url package](https://golang.org/pkg/net/url/#URL)
The general form represented is:

[scheme:][//[userinfo@]host][/]path[?query][#fragment]

URLs that do not start with a slash after the scheme are interpreted as:

scheme:opaque[?query][#fragment]

type URL struct {
    Scheme      string
    Opaque      string    // encoded opaque data
    User        *Userinfo // username and password information
    Host        string    // host or host:port
    Path        string    // path (relative paths may omit leading slash)
    RawPath     string    // encoded path hint (see EscapedPath method); added in Go 1.5
    ForceQuery  bool      // append a query ('?') even if RawQuery is empty; added in Go 1.7
    RawQuery    string    // encoded query values, without '?'
    Fragment    string    // fragment for references, without '#'
    RawFragment string    // encoded fragment hint (see EscapedFragment method); added in Go 1.15
}

### Flag package in go - stringparse
* [Building a Simple CLI Tool with Golang
](https://blog.rapid7.com/2016/08/04/build-a-simple-cli-tool-with-golang/)

### What is Flags in golang's definition?
* In the context of its flag package,
* all strings beginning with dashes (single or double) are considered flags,
* e.g. `-p`
* and all strings without dashes that are not user input are considered arguments.
* All strings after the first argument are considered arguments.
### Go by Example: Command-Line Flags
* :question: What does `flag.String()` do?
:::spoiler

    func String(name string, value string, usage string) *string

        String defines a string flag with specified name, default value, and usage
        string. The return value is the address of a string variable that stores the
        value of the flag.

:::
* Positional argument:
* all flags need to appear before positional arguments
* 因為 go 用的不是 `gnu style` flag
* 要使用 `gnu style flag`:
* 請用 [spf13 / pflag package](github.com/spf13/pflag)
* e.g.
* `./command-line-flags -word=opt a1 a2 a3 -numb=7`
* output:

                word: opt
                numb: 42
                fork: false
                svar: bar
                tail: [a1 a2 a3 -numb=7]

* :question: What does `flag.StringVar()` do ?
* It can pass the value from the flag to internal variable of program
### Go by Example: Command-Line Subcommands
* [Go by Example: Command-Line Subcommands](https://gobyexample.com/command-line-subcommands)
* :question: What is subcommand?
* e.g. `docker run -p 80:8080`
* `docker` is a command
* `run` is a subcommand
* `-p` is a flag for subcommand `run`

## Parser
### [Building a JSON Parser and Query Tool with Go](https://medium.com/@bradford_hamilton/building-a-json-parser-and-query-tool-with-go-8790beee239a)

### [Math parser in go](https://www.mathparsers.com/math-parser-for-go/)

### 9cc Compiler
[創造計算機等級的語言](https://koshizuow.gitbook.io/compilerbook/calculator_level_language)
:::success
* Finished:
* Makefile
* test.sh
* 9cc.c

TODO: [第2步：製作可以算加減法的編譯器](https://koshizuow.gitbook.io/compilerbook/calculator_level_language/step2)
:::
* :question: Return value of function 通常存在哪裡？
* ans: rax register
* Quote:把回傳值放進 RAX 是一般的共識
* ref: [簡單的範例](https://koshizuow.gitbook.io/compilerbook/machine_code_assembler/c_assembly/kantan_na_rei)
* :question: How to implement addition?

* :question: `strtol(p, &p, 10)` 在 p 指向 digit 的行為是？
* e.g. `5 + 20 + 4`, p point to addr of 4
* 會把 p 指向的 digit character 轉換成 long int 並 return,
* 並把 4 之後的 `\0` 的 addr assign 給 `p`
* 行為就像是 `p = p + 1` 一樣
* ref: man page of strtol
### Calculator
* Ref
* [rezroo/Dockerfile](https://gist.github.com/rezroo/da8c7c6169dc61da786e3ede78261854)

### Code

package main
import (
	"errors"
	"fmt"
	"os"
	"strconv"
)

func calculate(e []string) (float64, error) {
	result := 0.0
	num1, err := strconv.ParseFloat(e[0], 64)
	if err != nil {
		return 0.0, err
	}
	num2, err := strconv.ParseFloat(e[2], 64)
	if err != nil {
		return 0.0, err
	}
    switch e[1] {
    case "*":
        result = num1 * num2
    case "/":
        if num2 == 0.0 {
            return 0.0, errors.New("error: you tried to divide by zero.")
        }
        result = num1 / num2
    case "+":
        result = num1 + num2
    case "-":
        result = num1 - num2
    default:
        return 0.0, errors.New("error: don't know how to do that")
    }
	return result, nil
}

func main() {
    if len(os.Args) != 4 {
        fmt.Println("error: wrong number of arguments")
        return
    }
    res, err := calculate(os.Args[1:])
    if err != nil {
        fmt.Println(err)
    } else {
        fmt.Println(res)
    }
}