mirror of https://github.com/docker/cli.git
123 lines
3.2 KiB
Go
123 lines
3.2 KiB
Go
package template
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import (
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"fmt"
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"regexp"
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"strings"
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)
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var delimiter = "\\$"
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var substitution = "[_a-z][_a-z0-9]*(?::?[-?][^}]*)?"
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var patternString = fmt.Sprintf(
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"%s(?i:(?P<escaped>%s)|(?P<named>%s)|{(?P<braced>%s)}|(?P<invalid>))",
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delimiter, delimiter, substitution, substitution,
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)
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var pattern = regexp.MustCompile(patternString)
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// InvalidTemplateError is returned when a variable template is not in a valid
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// format
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type InvalidTemplateError struct {
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Template string
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}
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func (e InvalidTemplateError) Error() string {
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return fmt.Sprintf("Invalid template: %#v", e.Template)
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}
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// Mapping is a user-supplied function which maps from variable names to values.
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// Returns the value as a string and a bool indicating whether
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// the value is present, to distinguish between an empty string
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// and the absence of a value.
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type Mapping func(string) (string, bool)
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// Substitute variables in the string with their values
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func Substitute(template string, mapping Mapping) (string, error) {
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var err error
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result := pattern.ReplaceAllStringFunc(template, func(substring string) string {
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matches := pattern.FindStringSubmatch(substring)
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groups := matchGroups(matches)
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if escaped := groups["escaped"]; escaped != "" {
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return escaped
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}
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substitution := groups["named"]
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if substitution == "" {
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substitution = groups["braced"]
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}
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switch {
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case substitution == "":
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err = &InvalidTemplateError{Template: template}
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return ""
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// Soft default (fall back if unset or empty)
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case strings.Contains(substitution, ":-"):
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name, defaultValue := partition(substitution, ":-")
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value, ok := mapping(name)
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if !ok || value == "" {
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return defaultValue
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}
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return value
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// Hard default (fall back if-and-only-if empty)
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case strings.Contains(substitution, "-"):
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name, defaultValue := partition(substitution, "-")
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value, ok := mapping(name)
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if !ok {
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return defaultValue
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}
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return value
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case strings.Contains(substitution, ":?"):
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name, errorMessage := partition(substitution, ":?")
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value, ok := mapping(name)
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if !ok || value == "" {
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err = &InvalidTemplateError{
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Template: fmt.Sprintf("required variable %s is missing a value: %s", name, errorMessage),
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}
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return ""
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}
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return value
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case strings.Contains(substitution, "?"):
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name, errorMessage := partition(substitution, "?")
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value, ok := mapping(name)
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if !ok {
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err = &InvalidTemplateError{
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Template: fmt.Sprintf("required variable %s is missing a value: %s", name, errorMessage),
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}
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return ""
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}
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return value
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}
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value, _ := mapping(substitution)
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return value
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})
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return result, err
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}
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func matchGroups(matches []string) map[string]string {
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groups := make(map[string]string)
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for i, name := range pattern.SubexpNames()[1:] {
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groups[name] = matches[i+1]
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}
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return groups
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}
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// Split the string at the first occurrence of sep, and return the part before the separator,
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// and the part after the separator.
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//
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// If the separator is not found, return the string itself, followed by an empty string.
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func partition(s, sep string) (string, string) {
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if strings.Contains(s, sep) {
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parts := strings.SplitN(s, sep, 2)
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return parts[0], parts[1]
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}
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return s, ""
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}
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