aardvark.code

This go application reads in the user supplied definition (or structure) of a CSV file, and generates 'reader.go', a fully working go application that reads that CSV file. It does this using the nifty template feature of Go.

If you want to customize the resulting code, then just change the template reader.tpl. Or for a another CSV file, just change the description.txt.

Prerequisite

To run this aardvark.code example you need to have following software installed on your system:

Go aardvark

Execute

Stage 1

Read a .txt file and turn it into a Go Descriptor object which contains a Col object per column definition.

EntityName:City
0  Geonameid int
1  Name  string 
2  Asciiname string 
4  Lat float64      # latitude
5  Lon float64      # longitude
8  Country string 
14 Population int
16 Elevation float64

Filename:/u01/data/20150102_cities/cities1000.txt
Separator:\t

Stage 2

Read in the template 'reader.tpl', apply above data to it, and put the result in the file 'reader.go'.

I'm just going to lift the veil a bit, by showing what the following snippet of the template does, for more information about Go's template feature refer to the documentation: golang.org/pkg/text/template

For your own enlightenment, look for other «...» expressions in the 'reader.tpl' file, and see the code it generates in 'reader.go'.

Stage 3

Compile the 'reader.go' file, which reads in the CSV file, and shows a couple of records.

The aardvark.code

##== tmp/description.txt ========================================

EntityName:City
0  Geonameid int
1  Name  string 
2  Asciiname string 
4  Lat float64      # latitude
5  Lon float64      # longitude
8  Country string 
14 Population int
16 Elevation float64

Filename:/u01/data/20150102_cities/cities1000.txt
Separator:\t

##== tmp/reader.tpl ========================================

package main 

import ( 
    "os"
    "strconv"
    "bufio"
    "fmt"
    "io"
    "strings"
) 

func main() { 

    filename:="«.Filename»"
    f,err := os.Open(filename)
    defer f.Close()
    if err != nil {
        fmt.Fprintf(os.Stderr, "Opening file %q: %s\n", filename,err.Error())
        os.Exit(1) 
    }
    r:=bufio.NewReader(f)
    repeat:=true
    
    ignoredLines:=0
    list:=make([]«.EntityName»,0,0) 
    for repeat {
        line,overflow,err := r.ReadLine()
        repeat = (err!=io.EOF) // EOF means stop repeating this loop
        if err != nil && err!=io.EOF {
            fmt.Fprintf(os.Stderr, "Read error: %s\n", err.Error())
            break
        }
        if overflow {
            fmt.Fprintf(os.Stderr, "Overflow error on reading!\n")
            break
        }
        recs:=strings.Split(string(line),"«.Separator»")
        if len(recs)>«.Numcols» { 
            row,err:=extract( strings.Split(string(line),"«.Separator»"))
            if err!=nil { 
                // assume error already reported
                break
            }
            list=append(list,row)
        } else { 
            ignoredLines+=1
        }
    }
    if ignoredLines>0 { 
        fmt.Fprintf(os.Stderr, "Warning: %v line(s) ignored because of too few fields.\n",ignoredLines)  
    }
    for i,r:=range(list) { 
        fmt.Printf("%v\n",r)
        if i>10 { 
            break
        }
    }
}

type «.EntityName» struct { 
«range .Cols»    «.Identifier» «.Type»
«end»}

func extract(rec []string) (record «.EntityName»,err error) { 
«range .Cols»«if .ConversionFlag»«template "convert" .»«else»    _«.Identifier» := rec[«.Position»]«end»
«end»
    record = «.EntityName»{ «range .Cols»«.Identifier»:_«.Identifier», «end»}
    return
}

«define "convert"»«if eq .Type "int"»    _«.Identifier»:=0
    if len(rec[«.Position»])>0 {
        _«.Identifier»,err=strconv.Atoi(rec[«.Position»])
        if err != nil {
            fmt.Fprintf(os.Stderr, "Error converting «.Identifier»: %v\n", err.Error())
            return
        }
    }«end»«if eq .Type "float64"»    _«.Identifier»:=0.0
    if len(rec[«.Position»])>0 {
        _«.Identifier»,err=strconv.ParseFloat(rec[«.Position»],64)
        if err != nil {
            fmt.Fprintf(os.Stderr, "Error converting «.Identifier»: %v\n", err.Error())
            return
        }
    }«end»«end»


##== tmp/grok.go ========================================
package main 

import (
    "fmt"
    "bufio"
    "os"
    "regexp"
    "io/ioutil" 
    "strings"
    "strconv"
    "text/template"
)

type Descriptor struct { 
    EntityName string
    Filename string
    Separator string
    Numcols int
    Cols []Col    
}

type Col struct {
    Position int
    Identifier  string
    Type string
    ConversionFlag bool
}

func main() {
    d,err:=getDescriptor("tmp/description.txt")     // read the description 
    if err != nil {
        os.Exit(1) 
    }
fmt.Printf("%+v\n",d)   // DROPME TODO 
    f,err:=os.Create("tmp/reader.go")               // prepare file for output
    if err != nil {
        fmt.Fprintf(os.Stderr, "File open error: %s\n", err.Error())
        os.Exit(1) 
    }
    defer f.Close()
    w:=bufio.NewWriter(f)

    t:=template.New("reader.tpl")                   // create template 
    t.Delims("«","»")
    t=template.Must(t.ParseFiles("tmp/reader.tpl")) 
    err=t.Execute(w,d)                              // execute the template
    if err!=nil {
        fmt.Fprintf(os.Stderr, "Template execute error: %s\n", err.Error())
    }
    w.Flush()
}


func getDescriptor(filename string) (desc Descriptor, err error) { 
    desc=Descriptor{ EntityName:"x" } 

    content, err := ioutil.ReadFile(filename)
    if err != nil {
        fmt.Fprintf(os.Stderr, "File read error: %s\n", err.Error())
    }
    body:=strings.TrimSpace(strings.Replace( string(content), "\n","|",-1) )

    // regular expressions matching 1) key:value pair 2) fields line
    reKeyValue := regexp.MustCompile(`^\s*(\w+)\s*:\s*(\S+).*`)
    reFields   := regexp.MustCompile(`^\s*(\d+)\s*(\w+)\s*(\w+).*`)

    desc.Cols = make([]Col, 0, 0)
    for _,line:= range strings.Split(body,"|") { 
        if n:=strings.Index(line,"#"); n>-1 {       // remove comments
            line=line[:n]
        }
        line=strings.TrimSpace(line)                // empty string? 
        if len(line)<=1 { 
            continue
        } 
        group:=reKeyValue.FindStringSubmatch(line)  // pattern: key:value
        if group!=nil { 
            digestKeyValue(&desc, group) 
            continue    
        } 
        group=reFields.FindStringSubmatch(line)     // pattern: num word word 
        if group!=nil { 
            err=digestFields(&desc, group)
            if err!=nil { 
                break
            }
        }
    }
    desc.Numcols=len(desc.Cols)  
    return 
}

func digestKeyValue(desc *Descriptor, group []string) { 
    k:= group[1]
    v:= group[2] 
    if (k=="EntityName") { 
        desc.EntityName=v
    } else if (k=="Filename") { 
        desc.Filename=v
    } else if (k=="Separator") { 
        desc.Separator=v
    } else { 
        fmt.Fprintf(os.Stderr, "WARNING: Key:Value pair %v:%v ignored\n", k,v) 
    }
}

func digestFields(desc *Descriptor, group []string) (err error) { 
    p,err:=strconv.Atoi(group[1])
    if err != nil {
        fmt.Fprintf(os.Stderr, "Conversion error: %s\n", err.Error())
        return 
    }
    id:=group[2]
    desc.Cols=append(desc.Cols, Col{ Position:p, 
                                   Identifier:id,
                                   Type: group[3],
                                   ConversionFlag: group[3]!="string" })
    return 
}



##== aardvark.sh ========================================
#!/bin/bash 

rm grok tmp/reader.go reader    # cleanup 

go build tmp/grok.go            # build the code-generator 

if [ -x ./grok ]
then
    ./grok 
fi

if [ -f ./tmp/reader.go ]
then
    go build tmp/reader.go     # build the generated go code
    ./reader                   # and execute
fi

Generate Go code using templates