Five years ago indie rockers Razorlight headlined this festival in front of a crowd stretching right up to the burger bars. A few weeks ago, initial sales of singer Johnny Borrell's solo album stood at 594, prompting his label, Stiff, to waggishly proclaim it had managed "0.00015% the sales of Adele's 21".
The Borrell saga is a microcosm of why this year's Reading and Leeds festival is undergoing a watershed similar to that in 1988, when it shifted from rock to indie after Meat Loaf was pelted with bottles of pee. With even some of the more hotly tipped indie rockers here left twanging their guitars to tiny audiences, the promoters have bowed to the prevailing wind and dance acts made up a significant part of this year's bill.
Tents fill for AlunaGeorge's airy electro, Skrillex's laser-flashing dubstep and Major Lazer's dance hall party, which becomes uniquely thrilling when leader Diplo abseils over the crowd in a transparent plastic zorb while wearing fetching plastic wellies. Meanwhile, on the main stage, the brooding rage of festival staple Nine Inch Nails is perhaps further heightened by the sight of kids flocking away to watch bleep duo Disclosure.
Another development is the number of female acts at what has always been a predominantly male bastion. There's much love for Haim's slinky funky pop and Chvrches' Blondie-ish electro, while a dry tent during a downpour perhaps helps a mysterious sudden surge of interest in Kate Nash's unlikely switch from piano-tinkling chart pop to feminist punk racket.
While the festival may be witnessing various shifts in direction, there will always be a warm welcome for guitar bands who soundtrack the act of chucking cider in the air, especially when – like Friday headliners Biffy Clyro – they add the unique spectacle of bare-chested Scotsmen braving a torrential monsoon. Green Day top Saturday's bill with a two-hour masterclass that combines punk, call-and-response showmanship, and polemic. The ageing punks provide a particularly ludicrous but brilliant moment, when singer Billie Joe Armstrong concludes an epic rant about government lies and corruption with a shout of "Everyone in the field say 'Aay-ohh!'"
By Sunday, the muddy site bears witness to an epic, increasingly fascinating battle between genres. Guitar hero Johnny Marr unites the generations when his resplendent set ends in a mass singalong of Smiths classics. Foals demonstrate that guitars can make dance music too, and make a cheeky bid for universal appeal by dedicating their terrific set to "everyone on a three-day bender, all the single mums".
With guitars edging ahead, Chase & Status become the first dance act to brave the main stage since the Prodigy in 2009. The oddest thing about their demented, hugely popular rave-rock-house mix-up is their audience's behaviour, ranging from a spot of jousting with plastic penises to a girl who attempts a poo on a picture of indie star Jake Bugg.
Rapper Eminem unites the factions and pulls the biggest crowd for years. Backed by (yup) rock musicians, his set dips into pop and AOR – when Dido joins for the still disturbing Stan. Drugs and rehab have dulled his flow in recent years and Survival is the only new song aired, but the 40-year old is back fighting for his title as king rapper as fiercely as he once did as Jimmy Smith Jr in the fictional film 8 Mile. By the time My Name Is and a brutal Lose Yourself turn a muddy field into a hip-hop party, this festival might never be the same again.