1:- module(amazon_api, [ build_agent/2 2 , item_lookup/3 3 , item_lookup/4 4 , request/5 5 6 , offer_inventory/3 7 , offer_low_price/3 8 , title/2 9 ]). 10 11:- use_module(library(base64), [base64/2]). 12:- use_module(library(error), [existence_error/2]). 13:- use_module(library(func)). 14:- use_module(library(http/http_open), [http_open/3]). 15:- use_module(library(sha), [hmac_sha/4]). 16:- use_module(library(uri), [uri_encoded/3]). 17:- use_module(library(uri_qq)). 18:- use_module(library(xpath)).
associate_tag - stringaccess_key - stringsecret_key - string
It may include an optional host key which defaults
to "webservices.amazon.com".
32build_agent(Agent,Args) :- 33 must_have(Args, associate_tag, string, Tag), 34 must_have(Args, access_key, string, AccessKey), 35 must_have(Args, secret_key, string, SecretKey), 36 once( get_dict(host, Args, Host) 37 ; Host = "webservices.amazon.com" 38 ), 39 Agent = agent(Host,Tag,AccessKey,SecretKey). 40 41must_have(Args,Key,Type,Value) :- 42 once( get_dict(Key, Args, Value) 43 ; existence_error(Type, Key) 44 ). 45 46 47% private accessors for agent components 48agent_host(agent(Host,_,_,_),Host). 49agent_tag(agent(_,Tag,_,_),Tag). 50agent_key(agent(_,_,Key,_),Key). 51agent_secret(agent(_,_,_,Secret),Secret).
item_lookup(Agent,'B00004TN1Z',Item, _{'ResponseGroup':"Offers"}).
63item_lookup(Agent, Asin, Item, Args0) :-
64 Args = Args0.put(_{ 'ItemId' : Asin
65 , 'IdType' : "ASIN"
66 }),
67 request(Agent, get, "ItemLookup", Item, Args).72item_lookup(Agent,Asin,Item) :- 73 item_lookup(Agent,Asin,Item,_{}). 74 75 76%% request(+Agent,+Method:atom,+Operation:string,-Result,+Args:dict) 77% 78% Low level predicate for performing Amazon API requests. Executes 79% Operation using HTTP Method. API arguments are provided in 80% Args. On success, a parsed representation of the response XML 81% is bound to Result. See library(sgml) for the format. 82% 83% library(xpath) is helpful for extracting details from Result if 84% there's not already a helper predicate to handle it. 85request(Agent, Method, Operation, Result, Args0) :- 86 agent_tag(Agent, Tag), 87 agent_key(Agent, Key), 88 Args1 = Args0.put([ 'Service' = "AWSECommerceService" 89 , 'Version' = "2011-08-01" 90 , 'Operation' = Operation 91 , 'AWSAccessKeyId' = Key 92 , 'AssociateTag' = Tag 93 ]), 94 sign_request(Agent, Method, Args1, Args), 95 96 agent_host(Agent, Host), 97 Url = {|uri||http://$Host/onca/xml?$Args|}, 98 url_dom(Url, Result). 99 100 101% add timestamp and signature to Args0 giving Args 102sign_request(Agent, Method, Args0, Args) :- 103 % what string should we sign? 104 Args1 = Args0.put('Timestamp', timestamp(~)), 105 agent_host(Agent, Host), 106 signable_string(Method, Host, Args1, SignMe), 107 108 % perform the signature 109 agent_secret(Agent, Secret), 110 hmac_sha(Secret, SignMe, HmacBytes, [algorithm(sha256)]), 111 base64(string_codes(~,HmacBytes), Signature), 112 Args = Args1.put('Signature', Signature). 113 114 115% string to be signed to generate request signature 116signable_string(Method, Host, Args, String) :- 117 encode_pairs(Args, EncodedPairs), 118 String = "~s~n~s~n~s~n~s" $ [ upcase_atom $ Method 119 , Host 120 , "/onca/xml" 121 , EncodedPairs 122 ]. 123 124% encode request parameters into Amazon's canonical signature format 125encode_pairs(Dict, Atom) :- 126 dict_pairs(Dict, _, Pairs), 127 keysort(Pairs, Sorted), 128 maplist(encode_value, Sorted, Encoded), 129 atomic_list_concat(Encoded, "&", Atom). 130 131 132encode_value(K-V0,String) :- 133 uri_encode(V0, V), 134 String = "~s=~s" $ [K,V]. 135 136 137% encode URI values as Amazon expects. 138% uri_encoded/3 doesn't encode comma, :, / or ? characters. 139uri_encode(Value, Encoded) :- 140 uri_encoded(query_value, Value, E0), 141 atom_codes(E0, E1), 142 once(phrase(enc, E1, E2)), 143 atom_codes(Encoded, E2). 144 145enc, "%2C" --> ",", enc. 146enc, "%2F" --> "/", enc. 147enc, "%3A" --> ":", enc. 148enc, "%3F" --> "?", enc. 149enc, [C] --> [C], enc. 150enc --> { true }.
156timestamp(T) :-
157 get_time(Now),
158 stamp_date_time(Now, DT, 'UTC'),
159 format_time(string(T), "%FT%T.000Z", DT).165url_dom(Url, Dom) :- 166 setup_call_cleanup( http_open(Url, In, [timeout(10)]) 167 , url_dom_(Dom, In) 168 , close(In) 169 ). 170 171url_dom_(Dom, Stream) :- 172 load_structure( Stream 173 , [Dom|_] 174 , [ dialect(xml) 175 , max_errors(-1) 176 , syntax_errors(quiet) 177 ] 178 ). 179 180 181%% offer_inventory(+Item,?Condition:atom,-Inventory:integer) 182% 183% Find how many offers in Condition are available. Item is a value 184% produced by item_lookup/3, which must include the response group 185% "OfferSummary" (directly or indirectly). 186% 187% Condition should be one of `used`, `new` or `collectible`. Leaving 188% Condition unbound iterates all conditions on backtracking. 189offer_inventory(Item, Condition, Inventory) :- 190 inventory_node(Condition, InventoryNode), 191 once(xpath(Item, //'OfferSummary'/InventoryNode, Total)), 192 Total = element(_,_,[InventoryAtom]), 193 atom_number(InventoryAtom, Inventory). 194 195inventory_node(used, 'TotalUsed'). 196inventory_node(new, 'TotalNew'). 197inventory_node(collectible, 'TotalCollectible'). 198 199 200%% offer_low_price(+Item, ?Condition, -Pennies:integer) 201% 202% Find the lowest price for an offer in Condition. The price is 203% represented as an integer number of pennies, when using US Dollars. 204% An equivalent unit is used for other currencies. Item is a value 205% produced by item_lookup/3, which must include the response group 206% "OfferSummary" (directly or indirectly). 207% 208% Condition should be one of `used`, `new` or `collectible`. Leaving 209% Condition unbound iterates all conditions on backtracking. 210offer_low_price(Item, Condition, Pennies) :- 211 price_node(Condition, PriceNode), 212 once(xpath(Item, //'OfferSummary'/PriceNode/'Amount'(text), PriceAtom)), 213 atom_number(PriceAtom, Pennies). 214 215price_node(used, 'LowestUsedPrice'). 216price_node(new, 'LowestNewPrice'). 217price_node(collectible, 'LowestCollectiblePrice'). 218 219 220%% title(+Item, -Title:atom) is det. 221% 222% An item's title. Item is a value from item_lookup/3 and requires the 223% "ItemAttributes" response group. 224title(Item, Title) :- 225 once(xpath(Item, //