aardvark.code
 
11_code_gen
20161020

Generate Go code using templates

What?

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

Grab this aardvark.code file:

wget http://data.munging.ninja/aardvarkcode/codegen/aardvark.code

Execute

Stage 1

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

The structure as defined in the description.txt file

3
4
5
6
7
8
9
10
11
12
13
14
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

.. will look like this Go Descriptor object

{ 
    EntityName:City 
    Filename:/u01/data/20150102_cities/cities1000.txt 
    Separator:\t 
    Numcols:8 
    Cols:[
        {Position:0  Identifier:Geonameid  Type:int     ConversionFlag:true } 
        {Position:1  Identifier:Name       Type:string  ConversionFlag:false} 
        {Position:2  Identifier:Asciiname  Type:string  ConversionFlag:false} 
        {Position:4  Identifier:Lat        Type:float64 ConversionFlag:true } 
        {Position:5  Identifier:Lon        Type:float64 ConversionFlag:true } 
        {Position:8  Identifier:Country    Type:string  ConversionFlag:false} 
        {Position:14 Identifier:Population Type:int     ConversionFlag:true } 
        {Position:16 Identifier:Elevation  Type:float64 ConversionFlag:true }
    ]
}

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

When you apply the data to this template snipppet ..

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

.. this output will be produced:

type City struct { 
    Geonameid int
    Name string
    Asciiname string
    Lat float64
    Lon float64
    Country string
    Population int
    Elevation float64
}

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

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
##== 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
 
Notes by Data Munging Ninja. Generated on akalumba:sync/20151223_datamungingninja/aardvarkcode at 2018-02-24 12:57