refactor: taskfile/ast package (#1450)

* refactor: ast package

* feat: read -> taskfile

* refactor: taskfile.Taskfile -> taskfile.Read

* refactor: move merge function back into taskfile package

* refactor: rename taskfile.go to read.go

taskfile/ast/platforms.go

@@ -0,0 +1,100 @@
+package ast
+
+import (
+	"fmt"
+	"strings"
+
+	"gopkg.in/yaml.v3"
+
+	"github.com/go-task/task/v3/internal/goext"
+)
+
+// Platform represents GOOS and GOARCH values
+type Platform struct {
+	OS   string
+	Arch string
+}
+
+func (p *Platform) DeepCopy() *Platform {
+	if p == nil {
+		return nil
+	}
+	return &Platform{
+		OS:   p.OS,
+		Arch: p.Arch,
+	}
+}
+
+type ErrInvalidPlatform struct {
+	Platform string
+}
+
+func (err *ErrInvalidPlatform) Error() string {
+	return fmt.Sprintf(`task: Invalid platform "%s"`, err.Platform)
+}
+
+// UnmarshalYAML implements yaml.Unmarshaler interface.
+func (p *Platform) UnmarshalYAML(node *yaml.Node) error {
+	switch node.Kind {
+	case yaml.ScalarNode:
+		var platform string
+		if err := node.Decode(&platform); err != nil {
+			return err
+		}
+		if err := p.parsePlatform(platform); err != nil {
+			return err
+		}
+		return nil
+	}
+	return fmt.Errorf("yaml: line %d: cannot unmarshal %s into platform", node.Line, node.ShortTag())
+}
+
+// parsePlatform takes a string representing an OS/Arch combination (or either on their own)
+// and parses it into the Platform struct. It returns an error if the input string is invalid.
+// Valid combinations for input: OS, Arch, OS/Arch
+func (p *Platform) parsePlatform(input string) error {
+	splitValues := strings.Split(input, "/")
+	if len(splitValues) > 2 {
+		return &ErrInvalidPlatform{Platform: input}
+	}
+	if err := p.parseOsOrArch(splitValues[0]); err != nil {
+		return &ErrInvalidPlatform{Platform: input}
+	}
+	if len(splitValues) == 2 {
+		if err := p.parseArch(splitValues[1]); err != nil {
+			return &ErrInvalidPlatform{Platform: input}
+		}
+	}
+	return nil
+}
+
+// parseOsOrArch will check if the given input is a valid OS or Arch value.
+// If so, it will store it. If not, an error is returned
+func (p *Platform) parseOsOrArch(osOrArch string) error {
+	if osOrArch == "" {
+		return fmt.Errorf("task: Blank OS/Arch value provided")
+	}
+	if goext.IsKnownOS(osOrArch) {
+		p.OS = osOrArch
+		return nil
+	}
+	if goext.IsKnownArch(osOrArch) {
+		p.Arch = osOrArch
+		return nil
+	}
+	return fmt.Errorf("task: Invalid OS/Arch value provided (%s)", osOrArch)
+}
+
+func (p *Platform) parseArch(arch string) error {
+	if arch == "" {
+		return fmt.Errorf("task: Blank Arch value provided")
+	}
+	if p.Arch != "" {
+		return fmt.Errorf("task: Multiple Arch values provided")
+	}
+	if goext.IsKnownArch(arch) {
+		p.Arch = arch
+		return nil
+	}
+	return fmt.Errorf("task: Invalid Arch value provided (%s)", arch)
+}