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

extern crate proc_macro;

/// Automatic implementation of `StateMachine` for a record struct where every field
/// is also a `StateMachine`.
#[proc_macro_derive(StateMachine)]
pub fn state_machine_derive(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
    impl_state_machine_derive(input.into()).into()
}

use proc_macro2::{Ident, TokenStream};
use quote::quote;
use syn::{ItemStruct, Type, Visibility};

struct Field<'a> {
    name: &'a Ident,
    ty: &'a Type,
    apply_variant: Ident,
    conflict_variant: Ident,

    transition_ty: TokenStream,
    map_fn_name: Ident,
}

impl<'a> Field<'a> {
    pub fn new(field: &syn::Field) -> Field {
        let name = &field
            .ident
            .as_ref()
            .expect("Could not extract field identifier.");
        let name_str = inflections::case::to_pascal_case(&name.to_string());
        let apply_variant = quote::format_ident!("Apply{}", name_str);
        let conflict_variant = quote::format_ident!("{}Conflict", name_str);
        let ty = &field.ty;
        let transition_ty = quote! {
            <#ty as StateMachine>::Transition
        };

        let map_fn_name = quote::format_ident!("map_{}", name.to_string());

        Field {
            name,
            ty: &field.ty,
            apply_variant,
            conflict_variant,
            transition_ty,
            map_fn_name,
        }
    }

    fn generate_accessor(&self, enum_name: &Ident) -> TokenStream {
        let Field {
            name,
            ty,
            map_fn_name,
            apply_variant,
            transition_ty,
            ..
        } = self;

        quote! {
            pub fn #name(&self) -> &#ty {
                &self.#name
            }

            pub fn #map_fn_name(&self, fun: impl FnOnce(&#ty) -> #transition_ty) -> #enum_name {
                #enum_name::#apply_variant(fun(&self.#name))
            }
        }
    }

    fn generate_enum_variant(&self) -> TokenStream {
        let Field {
            apply_variant, ty, ..
        } = self;
        quote! {
            #apply_variant(<#ty as StateMachine>::Transition),
        }
    }

    fn generate_conflict_variant(&self) -> TokenStream {
        let Field {
            conflict_variant,
            ty,
            ..
        } = self;
        quote! {
            #conflict_variant(<#ty as StateMachine>::Conflict),
        }
    }

    fn generate_transition_case(
        &self,
        transition_name: &Ident,
        conflict_name: &Ident,
    ) -> TokenStream {
        let Field {
            name,
            apply_variant,
            conflict_variant,
            ..
        } = self;
        quote! {
            #transition_name::#apply_variant(val) => {
                match self.#name.apply(val) {
                    Ok(v) => {
                        let mut new_self = self.clone();
                        new_self.#name = v;
                        Ok(new_self)
                    },
                    Err(e) => Err(#conflict_name::#conflict_variant(e))
                }
            },
        }
    }
}

fn generate_transform(enum_name: &Ident, fields: &[Field], visibility: &Visibility) -> TokenStream {
    let variants: TokenStream = fields
        .iter()
        .flat_map(Field::generate_enum_variant)
        .collect();

    quote! {
        #[derive(serde::Serialize, serde::Deserialize, Debug, Clone, PartialEq)]
        #visibility enum #enum_name {
            #variants
        }
    }
}

fn generate_conflicts(enum_name: &Ident, fields: &[Field], visibility: &Visibility) -> TokenStream {
    let variants: TokenStream = fields
        .iter()
        .flat_map(Field::generate_conflict_variant)
        .collect();

    quote! {
        #[derive(serde::Serialize, serde::Deserialize, Debug, Clone, PartialEq)]
        #visibility enum #enum_name {
            #variants
        }
    }
}

fn impl_state_machine_derive(input: TokenStream) -> TokenStream {
    let ast: ItemStruct = syn::parse2(input).expect("Should decorate a struct.");

    let name = &ast.ident;
    let transform_name = quote::format_ident!("{}Transform", name.to_string());
    let conflict_name = quote::format_ident!("{}Conflict", name.to_string());

    let fields: Vec<Field> = match &ast.fields {
        syn::Fields::Named(fields) => fields.named.iter().map(Field::new).collect(),
        _ => panic!("Only structs with named fields can derive StateMachine currently."),
    };

    let accessors: TokenStream = fields
        .iter()
        .flat_map(|e| Field::generate_accessor(e, &transform_name))
        .collect();

    let transition_cases: TokenStream = fields
        .iter()
        .flat_map(|e| Field::generate_transition_case(e, &transform_name, &conflict_name))
        .collect();

    let visibility = &ast.vis;
    let transform_enum = generate_transform(&transform_name, &fields, visibility);
    let conflict_enum = generate_conflicts(&conflict_name, &fields, visibility);

    quote! {
        impl aper::StateMachine for #name {
            type Transition = #transform_name;
            type Conflict = #conflict_name;

            fn apply(&self, transition: &Self::Transition) -> Result<Self, Self::Conflict> {
                match transition {
                    #transition_cases
                }
            }
        }

        impl #name {
            #accessors
        }

        #transform_enum

        #conflict_enum
    }
}